In railway signalling, an interlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such as junctions or crossings. The signalling appliances and tracks are sometimes collectively referred to as an interlocking plant. An interlocking is designed so that it is impossible to display a signal to proceed unless the route to be used is proven safe.

In North America, the official railroad definition of interlocking is: "An arrangement of signals and signal appliances so interconnected that their movements must succeed each other in proper sequence".[1]

Des Plaines interlocking tower
The tower and tracks at Deval interlocking, Des Plaines, Illinois, in 1993

Configuration and use

Pennsylvania Railroad, HOLMES Block Station modelboard and lever machine
A model board and lever machine

A minimal interlocking consists of signals, but usually includes additional appliances such as points and Facing Point locks (US: switches) and derails, and may include crossings at grade and movable bridges. Some of the fundamental principles of interlocking include:

  • Signals may not be operated to permit conflicting train movements to take place at the same time on set route.
  • Switches and other appliances in the route must be properly 'set' (in position) before a signal may allow train movements to enter that route.
  • Once a route is set and a train is given a signal to proceed over that route, all switches and other movable appliances in the route are locked in position until either
    • the train passes out of the portion of the route affected, or
    • the signal to proceed is withdrawn and sufficient time has passed to ensure that a train approaching that route has had opportunity to come to a stop before passing the signal.


Railway interlocking is of British origin, where numerous patents were granted. In June 1856, John Saxby received the first patent for interlocking switches and signals.[2][3] In 1868, Saxby (of Saxby & Farmer)[4] was awarded a patent for what is known today in North America as “preliminary latch locking”.[5][6] Preliminary latch locking became so successful that by 1873, 13,000 mechanical locking levers were employed on the London and North Western Railway alone.[6][7]

The first experiment with mechanical interlocking in the United States took place in 1875 by J. M. Toucey and William Buchanan at Spuyten Duyvil Junction in New York on the New York Central and Hudson River Railroad (NYC&HRR).[6][7][8] At the time, Toucey was General Superintendent and Buchanan was Superintendent of Machinery on the NYC&HRR. Toucey and Buchanan formed the Toucey and Buchanan Interlocking Switch and Signal Company in Harrisburg, Pennsylvania in 1878. The first important installations of their mechanism were on the switches and signals of the Manhattan Elevated Railroad Company and the New York Elevated Railroad Company in 1877-78.[6] Compared to Saxby's design, Toucey and Buchanans' interlocking mechanism was more cumbersome and less sophisticated, and so was not implemented very widely.[8] Union Switch & Signal bought their company in 1882.[8]

As technology advanced that served to augment the muscle strength of human beings the railway signaling industry looked to incorporate these new technologies into interlockings to increase the speed of route setting, the number of appliances controlled from a single point and to expand the distance that those same appliances could be operated from the point of control. The challenge facing the signal industry was achieving the same level of safety and reliability that was inherent to purely mechanical systems. An experimental hydro-pneumatic[9] interlocking was installed at the Bound Brook, New Jersey junction of the Philadelphia and Reading Railroad and the Lehigh Valley Railroad in 1884.[6][7] By 1891, there were 18 hydro-pneumatic plants, on six railroads, operating a total of 482 levers.[6] The installations worked, but there were serious defects in the design, and little saving of labour was achieved.

The inventors of the hydro-pneumatic system moved forward to an electro-pneumatic system in 1891 and this system, best identified with the Union Switch & Signal Company, was first installed on the Chicago and Northern Pacific Railroad at its drawbridge across the Chicago River.[7] By 1900, 54 electro-pneumatic interlocking plants, controlling a total of 1,864 interlocking levers, were in use on 13 North American railroads. This type of system would remain one of two viable competing systems into the future, although it did have the disadvantage of needing extra single-use equipment and requiring high maintenance.[7]

Interlockings using electric motors for moving switches and signals became viable in 1894, when Siemens in Austria installed the first such interlocking at Přerov (now in the Czech Republic).[10] Another interlocking of this type was installed in Westend near Berlin in 1896.[11] In North America, the first installation of an interlocking plant using electric switch machines was at Eau Claire, Wisconsin on the Chicago, St. Paul, Minneapolis and Omaha Railway in 1901, by General Railway Signal Company (GRS, now a unit of Alstom, headquartered in Levallois-Perret, near Paris).[7] By 1913, this type system had been installed on 83 railroads in 35 US States and Canadian Provinces, in 440 interlocking plants using 21,370 levers.[6]

Interlocking types

Interlockings can be categorized as mechanical, electrical (electro-mechanical or relay-based), or electronic/computer-based.

Mechanical interlocking

Interlocking machine locking bed
A view of the locking bed inside Deval Tower, Des Plaines, Illinois

In mechanical interlocking plants, a locking bed is constructed, consisting of steel bars forming a grid. The levers that operate switches, derails, signals or other appliances are connected to the bars running in one direction. The bars are constructed so that if the function controlled by a given lever conflicts with that controlled by another lever, mechanical interference is set up in the cross locking between the two bars, in turn preventing the conflicting lever movement from being made.

In purely mechanical plants, the levers operate the field devices, such as signals, directly via a mechanical rodding or wire connection. The levers are about shoulder height since they must supply a mechanical advantage for the operator. Cross locking of levers was effected such that the extra leverage could not defeat the locking (preliminary latch lock).

The first mechanical interlocking was installed in 1843 at Bricklayers' Arms Junction, England.[12]:7

Electro-mechanical interlocking

Power interlockings may also use mechanical locking to ensure the proper sequencing of levers, but the levers are considerably smaller as they themselves do not directly control the field devices. If the lever is free to move based on the locking bed, contacts on the levers actuate the switches and signals which are operated electrically or electro-pneumatically. Before a control lever may be moved into a position which would release other levers, a signal must be received from the field element that it has actually moved into the position requested. The locking bed shown is for a GRS power interlocking machine.

Relay interlocking

Relay room
Part of a relay interlocking using miniature plug-in relays

Interlockings effected purely electrically (sometimes referred to as "all-electric") consist of complex circuitry made up of relays in an arrangement of relay logic that ascertain the state or position of each signal appliance. As appliances are operated, their change of position opens some circuits that lock out other appliances that would conflict with the new position. Similarly, other circuits are closed when the appliances they control become safe to operate. Equipment used for railroad signalling tends to be expensive because of its specialized nature and fail-safe design.

Interlockings operated solely by electrical circuitry may be operated locally or remotely, with the large mechanical levers of previous systems being replaced by buttons, switches or toggles on a panel or video interface. Such an interlocking may also be designed to operate without a human operator. These arrangements are termed automatic interlockings, and the approach of a train sets its own route automatically, provided no conflicting movements are in progress.

GRS manufactured the first all-relay interlocking system in 1929. It was installed in Lincoln, Nebraska on the Chicago, Burlington and Quincy Railroad.[12]:18

Promenade St Tower Control Panel
Control panel for a US&S relay interlocking

Entrance-Exit Interlocking (NX) was the original brand name of the first generation relay-based centralized traffic control (CTC) interlocking system introduced in 1936 by GRS[13] (represented in Europe by Metropolitan-Vickers). The advent of all electric interlocking technology allowed for more automated route setting procedures as opposed to having an operator line each part of the route manually. The NX system allowed an operator looking at the diagram of a complicated junction to simply push a button on the known entrance track and another button on the desired exit track. The logic circuitry handled all the necessary actions of commanding the underlying relay interlocking to set signals and throw switches in the proper sequence, as required to provide valid route through the interlocking plant. The first NX installation was in 1937 at Brunswick on the Cheshire Lines, UK. The first US installation was on the New York Central Railroad (NYCRR) at Girard Junction, Ohio in 1937.[12]:18 Another NYCRR installation was on the main line between Utica, New York and Rochester, New York, and this was quickly followed up by three installations on the New York City Subway's IND Fulton Street Line in 1948.[14][15]

Other NX style systems were implemented by other railroad signal providers. For example, Union Route (UR) was the brand name of their Entrance-Exit system supplied by Union Switch & Signal Co. (US&S), and introduced in 1951.[16] NX type systems and their costly pre-solid state control logic only tended to be installed in the busier or more complicated terminal areas where it could increase capacity and reduce staffing requirements. In a move that was popular in Europe, the signalling for an entire area was condensed into a single large power signal box with a control panel in the operator's area and the equivalent of a telephone exchange in the floors below that combined the vital relay based interlocking logic and non-vital control logic in one place. Such advanced schemes would also include train describer and train tracking technologies. Away from complex terminals unit lever control systems remained popular until the 1980s when solid state interlocking and control systems began to replace the older relay plants of all types.

Electronic interlocking

Antwerpen Noord seinhuis
Computer-based controls for a modern electronic interlocking

Modern interlockings (those installed since the late 1980s) are generally solid state, where the wired networks of relays are replaced by software logic running on special-purpose control hardware. The fact that the logic is implemented by software rather than hard-wired circuitry greatly facilitates the ability to make modifications when needed by reprogramming rather than rewiring. In many implementations, this vital logic is stored as firmware or in ROM that cannot be easily altered to both resist unsafe modification and meet regulatory safety testing requirements.

At this time there were also changes in the systems that controlled interlockings. Whereas before technologies such as NX and Automatic Route Setting required racks and racks of relays and other devices, solid state software based systems could handle such functions with less cost and physical footprint. Initially processor driven Unit Lever and NX panels could be set up to command field equipment of either electronic or relay type; however as display technology improved, these hard wired physical devices could be updated with visual display units, which allowed changes in field equipment be represented to the signaller without any hardware modifications.

Solid State Interlocking (SSI) is the brand name of the first generation microprocessor-based interlocking developed in the 1980s by British Rail, GEC-General Signal and Westinghouse Signals Ltd in the UK. Second generation processor-based interlockings are known by the term "Computer Based Interlocking" (CBI),[17] of which VPI (trademark of General Railway Signal, now Alstom), MicroLok (trademark of Union Switch & Signal, now Ansaldo STS), Westlock and Westrace (trademarks of Invensys Rail, now Siemens), and [Smartlock[18]] (trademark of Alstom) are examples.

Defined forms of locking

Electric locking
"The combination of one or more electric locks or controlling circuits by means of which levers in an interlocking machine, or switches or other devices operated in connection with signalling and interlocking, are secured against operation under certain conditions."[19]
Section locking
"Electric locking effective while a train occupies a given section of a route and adapted to prevent manipulation of levers that would endanger the train while it is within that section."[19]
Route locking
"Electric locking taking effect when a train passes a signal and adapted to prevent manipulation of levers that would endanger the train while it is within the limits of the route entered."[19]
Sectional route locking
"Route locking so arranged that a train, in clearing each section of the route, releases the locking affecting that section."[19]
Electric railway lines
Electric railway lines
Approach locking
"Electric locking effective while a train is approaching a signal that has been set for it to proceed and adapted to prevent manipulation of levers or devices that would endanger that train."[19]
Stick locking
"Electric locking taking effect upon the setting of a signal for a train to proceed, released by a passing train, and adapted to prevent manipulation of levers that would endanger an approaching train."[19]
Indication locking
"Electric locking adapted to prevent any manipulation of levers that would bring about an unsafe condition in case a signal, switch, or other operated device fails to make a movement corresponding with that of the operating lever; or adapted directly to prevent the operation of one device in case another device, to be operated first, fails to make the required movement."[19]
Check locking or traffic locking
"Electric locking that enforces cooperation between the Operators at two adjacent plants in such a manner that prevents opposing signals governing the same track from being set to proceed at the same time. In addition, after a signal has been cleared and accepted by a train, check locking prevents an opposing signal at the adjacent interlocking plant from being cleared until the train has passed through that plant."[19]

Complete and incomplete interlockings (U.S. terminology)

Interlockings allow trains to cross from one track to another using a turnout and a series of switches. Railroad terminology defines the following types of interlockings as either complete or incomplete depending on the movements available. Although timetables generally do not identify an interlocking as one or the other, and rule books do not define the terms, the terms below are generally agreed upon by system crews and rules officials.

Complete interlockings
allow continuous movements from any track on one side of the interlocking to any track on the opposite side without the use of a reverse move within the limits of the interlocking. This is true even if there are differing numbers of tracks on opposing sides, or if the interlocking has multiple sides.
Incomplete interlockings
do not allow such movements as described above. Movements in an incomplete interlocking may be limited and may even require reverse movements to achieve the desired route.


  1. ^ Josserand, Peter; Forman, Harry Willard (1957). Rights of Trains (5th ed.). New York: Simmons-Boardman Publishing Corporation. p. 5. OCLC 221677266. Definitions.
  2. ^ "Death of John Saxby". Railway Age Gazette. Simmons-Boardman Publishing Corporation. 54 (20): 1102. 26 May 1913. OCLC 15110423.
  3. ^ Solomon, Brian (2003). Railroad Signaling. St Paul, Minnesota: MBI Publishing Company. pp. 23–24. ISBN 978-0-7603-1360-2. OCLC 52464704.
  4. ^ The first manufacturer of signal equipment, the predecessor of Westinghouse Brake and Signal Company Ltd, and today’s Westinghouse Rail Systems, Ltd. (headquartered in Chippenham, Wiltshire)
  5. ^ US patent 80878, John Saxby & John Stinson Farmer, "Improved Switch and Signal", issued 11 August 1868
  6. ^ a b c d e f g "Landmarks in Signaling History". Railway Age Gazette. Simmons-Boardman Publishing Corporation. 61 (4): 161. 28 July 1916.
  7. ^ a b c d e f General Railway Signal Company (1913). Sperry, Henry M., ed. Electric Interlocking Handbook. Rochester, New York: General Railway Signal Company. pp. 5–12. OCLC 3527846.
  8. ^ a b c Calvert, J. B. "Toucey and Buchanan Interlocking". Railways: History, Signalling, Engineering. Archived from the original on 23 April 2012. Retrieved 28 December 2011.
  9. ^ A system whereby compressed water and air are used to transmit action from one end of a long tube to the other end. It can be effective, but it still qualifies as a mechanical system since the pressure is pre-loaded, and requires human action of the same sort that a pure mechanical system requires.
  10. ^ Lexikon der gesamten Technik, entry "Stellwerke"
  11. ^ "Berliner Stellwerke". Archived from the original on 28 November 2012. Retrieved 24 November 2012.
  12. ^ a b c Alstom Signaling Incorporated (2004). A Centennial: History of Alstom Signaling Inc (PDF). West Henrietta, New York: Alstom. Archived from the original (PDF) on 2 October 2011. Retrieved 27 December 2011.
  13. ^ General Railway Signal Company (1936). The NX System of Electric Interlocking (PDF). Rochester, New York. OCLC 184909207. Archived (PDF) from the original on 2010-11-28.
  14. ^ "Signaling and Interlocking On New Line of New York Subways". Railway Signaling and Communications. Simmons-Boardman Publishing Corporation.: 578–583 September 1949. Retrieved 27 December 2016.
  15. ^ "Buttons to Speed Travel in Subway: $2,000,000 System of Signals Soon to Be in Operation on Brooklyn IND Division" (PDF). The New York Times. November 12, 1948. Archived (PDF) from the original on 12 February 2018. Retrieved 27 December 2016.
  16. ^ US patent 2567887, Ronald A. McCann, "Entrance-exit route interlocking control apparatus", issued 11 September 1951, assigned to The Union Switch and Signal Company
  17. ^ Woolford, Paul (April 2004). Glossary of Signalling Terms (PDF) (Report). Railway Group Guidance Note GK/GN0802. London: Rail Safety and Standards Board. Archived from the original (PDF) on 8 May 2016. Retrieved 20 April 2016.
  18. ^ "Smartlock Interlocking". Archived from the original on 8 October 2017. Retrieved 4 May 2018.
  19. ^ a b c d e f g h Defined by the Railway Signal Association, which today is the Railway Signal Committee of the Association of American Railroads.

External links

Crown of Thorns (woodworking)

The Crown of Thorns (puzzle work) is a woodworking technique of tramp art using interlocking wooden pieces that are notched to intersect at right angles forming joints and self-supporting objects, objects that have a "prickly" and transparent quality. Common examples include wreath-shaped picture frames that look similar to Jesus' "crown of thorns".

Larger-scale crowns may use the principles of tensegrity structures, where the wooden sticks provide rigidity and separate cables in tension carry the forces that hold them together.


Dougong (Chinese: 斗拱; pinyin: dǒugǒng; literally: "cap [and] block") is a unique structural element of interlocking wooden brackets, one of the most important elements in traditional Chinese architecture.

The use of dougong first appeared in buildings of the late centuries BC and evolved into a structural network that joined pillars and columns to the frame of the roof. Dougong was widely used in the ancient Chinese during the Spring and Autumn period (770–476 BC) and developed into a complex set of interlocking parts by its peak in the Tang and Song periods. The pieces are fitted together by joinery alone without glue or fasteners, due to the precision and quality of the carpentry.

After the Song Dynasty, brackets and bracket sets became more ornamental than structural when used in palatial structures and important religious buildings, no longer the traditional dougong.

Finger joint

A Finger Joint, also known as a Comb Joint, is a woodworking joint made by cutting a set of complementary, interlocking profiles in two pieces of wood, which are then glued. The cross-section of the joint resembles the interlocking of fingers between two hands, hence the name "Finger joint". The sides of each profile increases the surface area for gluing, resulting in a strong bond, stronger than a butt joint but not very visually appealing. Finger joints are regularly confused with box joints however, box joints are a more decorative but simple joint.

Girih tiles

Girih tiles are a set of five tiles that were used in the creation of Islamic geometric patterns using strapwork (girih) for decoration of buildings in Islamic architecture. They have been used since about the year 1200 and their arrangements found significant improvement starting with the Darb-i Imam shrine in Isfahan in Iran built in 1453.

Harold Interlocking

Harold Interlocking is a large railroad junction located in New York City. It is the busiest rail junction in the United States. It serves trains on Amtrak's Northeast Corridor and the Long Island Rail Road's Main Line and Port Washington Branch, which diverge at the junction.

Reconstruction work on Harold Interlocking started in 2009, as part of the East Side Access project to bring LIRR service to Grand Central Terminal. As part of the project, two tunnels for Northeast Corridor trains bypassing Harold Interlocking are being built to address congestion problems and occasional accidents.

Harrisburg Subdivision

The Harrisburg Subdivision is a railroad line owned by CSX Transportation in the U.S. state of Pennsylvania. The line is located in the city of Philadelphia, connecting Greenwich Yard and the Philadelphia Subdivision with the Trenton Subdivision along a former Pennsylvania Railroad line. Much of the Harrisburg Subdivision is the High Line or West Philadelphia Elevated along 31st Street over the 30th Street Station area.

The line begins at Greenwich Yard in South Philadelphia, where it meets the Philadelphia Belt Line Railroad. It heads west alongside the Delaware Expressway (Interstate 95) and then north along and partially elevated over 25th Street, turning west at Washington Avenue to cross the Schuylkill River on the Arsenal Bridge. At Arsenal Interlocking, on the west side of the Schuylkill, a branch runs southwest alongside Amtrak's Northeast Corridor to a junction with the Philadelphia Subdivision near Lindbergh Boulevard. The main line heads north from Arsenal, rises onto the elevated structure, and crosses to the west side of the Northeast Corridor. It heads north above 31st Street, finally touching down in the southeast approach to Zoo Interlocking. It leaves that interlocking to the north, staying on the west side of the Schuylkill, and ending at the Trenton Subdivision at Belmont.

Howell Interlocking Historic District

Howell Interlocking Historic District is the area in West Midtown, Atlanta where four railroad lines converge.

It is adjacent to the Marietta Street Artery neighborhood, an area rich in industrial history, as an original industrial district built along Atlanta's first railway line (1837).

Interlocking directorate

Interlocking directorate refers to the practice of members of a corporate board of directors serving on the boards of multiple corporations. A person that sits on multiple boards is known as a multiple director. Two firms have a direct interlock if a director or executive of one firm is also a director of the other, and an indirect interlock if a director of each sits on the board of a third firm. This practice, although widespread and lawful, raises questions about the quality and independence of board decisions.


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Lever frame

Mechanical railway signalling installations rely on lever frames for their operation to interlock the signals, track locks and points to allow the safe operation of trains in the area the signals control. Usually located in the signal box, the levers are operated either by the signalman or the pointsman.

The world's largest lever frame (191 levers) is thought to have been in the Spencer Street No.1 signal box in Melbourne, Australia, which was decommissioned in 2008. The largest operational lever frame, meanwhile, is located at Severn Bridge Junction in Shrewsbury, England, and it contains 180 levers, although most of them have been taken out of use.

Main Line (Long Island Rail Road)

The Main Line is a rail line owned and operated by the Long Island Rail Road in the U.S. state of New York. It begins as a two-track line at the Long Island City station in Long Island City, Queens, and runs along the middle of Long Island about 95 miles (153 km) to the Greenport station in Greenport, Suffolk County. A mile east of the Long Island City station (east of Hunterspoint Avenue), the four tracks of the East River Tunnels join the two tracks from Long Island City; most Main Line trains use those tunnels rather than running to or from Long Island City.

Continuing east, five branches split from the Main Line. In order from west to east, they are:

Port Washington Branch (at Harold Interlocking in Long Island City, Queens)

Hempstead Branch (at Queens Interlocking along the Queens/Nassau County border)

Oyster Bay Branch (at Nassau Interlocking, east of Mineola station)

Port Jefferson Branch (at Divide Interlocking, east of Hicksville station)

Central Branch (at Beth Interlocking, east of Bethpage station)—single non-electrified track with no stations, connecting the Main Line to the Montauk BranchThe Main Line contains four tracks between Harold and Queens Interlockings, and two tracks to the west and east of these points. Public timetables refer to the Main Line east of Hicksville as the Ronkonkoma Branch. The Ronkonkoma Branch continues eastward as a two-track electrified branch to Ronkonkoma station, where it narrows to one non-electrified track.

Trains on the Main Line between Long Island City and Ronkonkoma are governed by Automatic Block and Interlocking Signals and by Automatic Train Control (which the LIRR refers to as Automatic Speed Control). East of Ronkonkoma, trains operate in non-signaled dark territory, with all train movements being governed by train orders and timetable authority. Several projects are underway to expand the Main Line's train capacity, including the Third-Track project between New Hyde Park station and Divide Interlocking; the Double Track project along the Ronkonkoma Branch, completed in 2018; and the reconstruction of Harold Interlocking.

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Pavement comes from Latin pavimentum, meaning a floor beaten or rammed down, through Old French pavement. The meaning of a beaten-down floor was obsolete before the word entered English.Pavement laid in patterns like mosaics were commonly used by the Romans.

Roller coaster elements

Roller coaster elements are the individual parts of roller coaster design and operation, such as a track, hill, loop, or turn. Variations in normal track movement that add thrill or excitement to the ride are often called "thrill elements" or "thrill factor".

Signalling control

On a rail transport system, signalling control is the process by which control is exercised over train movements by way of railway signals and block systems to ensure that trains operate safely, over the correct route and to the proper timetable. Signalling control was originally exercised via a decentralised network of control points that were known by a variety of names including signal box (International and British), interlocking tower (North America) and signal cabin (some railways e.g., GCR). Currently these decentralised systems are being consolidated into wide scale signalling centres or dispatch offices. Whatever the form, signalling control provides an interface between the human signal operator and the lineside signalling equipment. The technical apparatus used to control switches (points), signals and block systems is called interlocking.

Solid State Interlocking

Solid State Interlocking (SSI) is the brand name of the first generation processor-based interlocking developed in the 1980s by British Rail's Research Division, GEC-General Signal and Westinghouse Signals Ltd in the UK.

Texas League

The Texas League is a Minor League Baseball league which operates in the South Central United States. It is classified as a Double-A league.

Despite the league's name, only its four South Division teams are actually based in Texas; the four North Division teams are located in surrounding states of Oklahoma, Arkansas, and Missouri. The league maintains its headquarters in San Antonio.

The league was founded in 1888 and ran through 1892. It was called the Texas Association in 1895, the Texas-Southern League in 1896 and again as the Texas League from 1897–1899. It was revived as a Class D league in 1902, moved to Class C in 1904 where it played through 1910 (except for 1906 as Class D again), played at Class B until 1920, and finally moved up to Class A in 1921. The Texas League, like many others, shut down during World War II. From 1959 to 1961, the Texas League and the Mexican League formed the Pan American Association. The two leagues played a limited interlocking schedule and post-season championship. By 1971, the Texas League and the Southern League had both decreased to seven teams. They played an interlocking schedule with the Southern League known as the Dixie Association. The two leagues played separate playoffs.

The League's name is well known due to its association with a particular aspect of the game. A bloop single that drops between the infielders and outfielders has been called a Texas Leaguer since the 1890s, despite no evidence that it originated in the Texas League, or was any more common there than elsewhere. There is a common thread throughout Civil War anecdotes that refer to a game played 30 years earlier in the Sabine Pass area. As the story goes, a Union soldier hit a ball over the outfielder's head, leading him into a long chase for the ball which resulted in a bullet wound from a nearby sniper. After the incident, hits were only awarded for balls that landed between the infielders and outfielders.

United States Senate Subcommittee on Internal Security

The Special Subcommittee to Investigate the Administration of the Internal Security Act and Other Internal Security Laws, 1951–77, more commonly known as the Senate Internal Security Subcommittee (SISS) and sometimes the McCarran Committee, was authorized under S. 366, approved December 21, 1950, to study and investigate (1) the administration, operation, and enforcement of the Internal Security Act of 1950 (Pub.L. 81–831, also known as the McCarran Act) and other laws relating to espionage, sabotage, and the protection of the internal security of the United States and (2) the extent, nature, and effects of subversive activities in the United States "including, but not limited to, espionage, sabotage, and infiltration of persons who are or may be under the domination of the foreign government or organization controlling the world Communist movement or any movement seeking to overthrow the Government of the United States by force and violence". The resolution also authorized the subcommittee to subpoena witnesses and require the production of documents. Because of the nature of its investigations, the subcommittee is considered by some to be the Senate equivalent to the older House Un-American Activities Committee (HUAC).

The chairman of the subcommittee in the 82nd United States Congress was Patrick McCarran of Nevada. William Jenner of Indiana took over during the 83rd United States Congress after the Republicans gained control of the Senate in the 1952 election. When the Democrats regained control in the 84th Congress (1955–1957), James O. Eastland of Mississippi became chairman, a position he held until the subcommittee was abolished in 1977.

The subjects of its investigations during the 1950s include the formulation of U.S. foreign policy in Asia; the scope of Soviet activity in the United States; subversion in the Federal Government, particularly in the Department of State and Department of Defense; immigration; the United Nations; youth organizations; the television, radio, and entertainment industry; the telegraph industry; the defense industry; labor unions; and educational organizations. In the 1960s, the investigations were expanded to include civil rights and racial issues, campus disorders, and drug trafficking. The subcommittee published over 400 volumes of hearings and numerous reports, documents, and committee prints.

The major classes of records of the subcommittee are the investigative and administrative records, and the special collections. There are also several smaller files. Due to the ongoing nature of the investigations, the investigative files were not maintained either by year or Congress; instead, individual files may contain information accumulated over a period of 20 or more years. It is impractical, therefore, to limit a description of the records of the subcommittee to those through 1968, and although the files were begun in 1951, some contain data that precedes the creation of the subcommittee.

In March 1951, FBI officials forged a formal liaison program with the SISS in contract to the informal HUAC-FBI relationship, whereby SISS agreed to focus its hearings on "matters of current internal security significance...[and also] to help the Bureau in every possible manner". Under this program, the SISS forwarded to the FBI any confidential information they uncovered and the FBI conducted name checks on prospective SISS witnesses, submitted reports on targeted organizations, and provided memoranda "with appropriate leads and suggested clues". This was all intended to avert the perception that HUAC's purpose was to discredit the loyalty of officials in the Roosevelt and Truman administrations. This program reflected the FBI director's unqualified confidence in McCarran's ability to serve the cause of anticommunism and to protect the confidentiality of FBI sources.The investigation of the Institute of Pacific Relations (IPR) was the first major investigation initiated by the subcommittee. Some people accused the IPR leadership of spying for the Soviet Union. Owen Lattimore, editor of the IPR journal Pacific Affairs, was especially singled out for criticism. It is also believed that the pressure of the investigation triggered the suicide of the UN Assistant Secretary General Abraham Feller on November 13, 1952.

To investigate these charges, the SISS took possession of the older files of the IPR, which had been stored at the Lee, Massachusetts farm of Edward C. Carter, an IPR trustee. The subcommittee's investigators studied these records for 5 months, then held hearings for nearly 1 year (July 25, 1951 – June 20, 1952). The final report of the subcommittee was issued in July 1952 (S. Rpt. 2050, 82d Cong., 2d sess., Serial 11574).

Westlock Interlocking

WESTLOCK Interlocking is a Solid State Interlocking (SSI) product by Westinghouse Rail Systems.

Westlock builds on many of the features that made SSI popular in the United Kingdom. This includes re-use of SSI's programming language and its external hardware.

In addition to backwards compatibility with SSI, Westlock provides additional capacity. This is currently estimated to be around four times that of an SSI, giving it the capability to control over 300 Signalling Equivalent Units (SEUs).

Leamington Spa was the test site for Westlock, and was its first operational site. It was also SSI's test site more than 20 years earlier.

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