Part of a railway signaling system, a slide fence is a fence whose purpose is to prevent trains from being derailed by rock slides in mountainous areas where rock slides may occur without warning. The fence is designed to be displaced by a rock slide, causing the signaling system to display a stop aspect on nearby signals. As an alternative, a structural fence is designed to physically stop falling rocks from reaching the tracks.
The mechanical slide fence consists of a series of tensioned wires strung about 10 inches (25 cm) apart on poles. When a rock slide occurs, the rock breaks one or more of the wires. When a wire breaks, heavy weights attached to either end will fall. This mechanically triggers the protecting signals to the 'danger' position.
There are two types of electrical slide fences in operation.
One type of electrical slide fence consists of a series of parallel conductive wires strung about 8 inches (20 cm) apart on poles that create a fence parallel to the rails.
This creates an electrical circuit that is monitored by signaling equipment. In normal operation, the electric current in the fence wires causes a relay to energize, indicating that the fence is intact. When a rock slide occurs, the rock breaks one or more of the wires, interrupting the current. This causes the relay to become de-energized, indicating that a rock slide has occurred. A contact of the relay is typically used to prevent the approaching signal from displaying a proceed aspect if the fence has been broken. Restoring normal operation requires splicing the broken fence wires back together.
Another type of slide fence is similar except that the wires do not have to break and is easier to maintain and reset. The slide fence consists of a series of fence sections, as shown in the SLIDE FENCE DETAIL drawing (pictured right), which shows a typical installation. Each fence section is held in place by strong springs. At each end of a fence section is an electro-mechanical plug. The plugs maintain a complete electrical circuit that is monitored by signaling equipment. In normal operation, the current through the plugs causes a relay to energize, indicating that the fence is in place. When a rock slide occurs, the fence moves laterally, causing the plug to be removed, breaking the circuit. This causes the relay to become de-energized, indicating that a rock slide has occurred. A contact of the relay is typically used to prevent the approaching signal from displaying a proceed aspect if the fence has been broken. Restoring normal operation requires re-inserting the plugs that were dislodged by the slide.
The structural fence is a physical barrier designed to stop falling rocks from reaching the tracks. Several construction methods are used, including: steel I-beams, wooden barriers, galvanized fencing, and netting directly against the rock.
A slide fence is typically found in mountainous areas in a rock cut area, where rocks may fall on the track and present a danger to passing trains. The length of the fence may range from 100 feet (30 meters) to several miles (kilometers), depending on the length of the rock cut and the area being protected. The slide fence is usually located on the uphill side of the track in the slide area.
Select examples of railroad slide fences:
When a train approaches a slide fence area, and the signal displays a stop aspect, the train is not permitted to proceed normally, because a rock slide has occurred. However, the rock slide may not prevent the safe passage of the train. For example, a large rock may have fallen off the cliff, broken through the slide fence, and continued to fall away from the track. After stopping, the train may obtain radio permission from the dispatch center to proceed slowly, watching for a dangerous rock slide. If the train is able to successfully pass through the slide area (that is, there is no danger), it may then be allowed to proceed normally.
In North America slide fences are typically connected in such a way as to shunt the track circuit when activated. This causes signals on either side of the slide fence to display a restricting indication, requiring trains to travel at a speed enabling them to stop within one half the range of vision. On lines formerly operated by the Pennsylvania Railroad signals connected to a slide detector have an 'SP' placard, reminding engineers to watch for slides when governed by a restrictive speed signal.
Once the slide fence has been activated (even if in error), all trains are affected until the slide fence is repaired by maintenance personnel. This may result in several hours of delay in train service.
Several alternative technologies have been tried to solve the rock slide problem, including:
The term landslide or less frequently, landslip, refers to several forms of mass wasting that include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows. Landslides occur in a variety of environments, characterized by either steep or gentle slope gradients, from mountain ranges to coastal cliffs or even underwater, in which case they are called submarine landslides. Gravity is the primary driving force for a landslide to occur, but there are other factors affecting slope stability that produce specific conditions that make a slope prone to failure. In many cases, the landslide is triggered by a specific event (such as a heavy rainfall, an earthquake, a slope cut to build a road, and many others), although this is not always identifiable.Pass of Brander stone signals
The Pass of Brander stone signals are a series of railway signals situated in the Pass of Brander, between Loch Awe and Taynuilt stations on the Oban branch of the West Highland Line in Scotland. They are part of a warning system that advises train drivers to exercise caution in the event of a rock-fall.Railway signalling
Railway signalling is a system used to direct railway traffic and keep trains clear of each other at all times. Trains move on fixed rails, making them uniquely susceptible to collision. This susceptibility is exacerbated by the enormous weight and inertia of a train, which makes it difficult to quickly stop when encountering an obstacle. In the UK, the Regulation of Railways Act 1889 introduced a series of requirements on matters such as the implementation of interlocked block signalling and other safety measures as a direct result of the Armagh rail disaster in that year.
Most forms of train control involve movement authority being passed from those responsible for each section of a rail network (e.g., a signalman or stationmaster) to the train crew. The set of rules and the physical equipment used to accomplish this determine what is known as the method of working (UK), method of operation (US) or safeworking (Aus.). Not all these methods require the use of physical signals, and some systems are specific to single track railways.
The earliest rail cars were first hauled by horses or mules. A mounted flagman on a horse preceded some early trains. Hand and arm signals were used to direct the “train drivers”. Foggy and poor-visibility conditions later gave rise to flags and lanterns. Wayside signalling dates back as far as 1832, and used elevated flags or balls that could be seen from afar.