railway signaling
Railway signalling (), also called railroad signaling (), is a system used to control the movement of railway traffic. Trains move on fixed rails, making them uniquely susceptible to collision. This susceptibility is exacerbated by the enorm ...
system, a slide fence is a fence whose purpose is to prevent trains from being derailed by
rockslide
A rockslide is a type of landslide caused by rock failure in which part of the bedding plane of failure passes through compacted rock and material collapses ''en masse'' and not in individual blocks. Note that a rockslide is similar to an avalanc ...
s in mountainous areas where rockslides 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.
Operation
Mechanical
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.
Electrical
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
An electrical network is an interconnection of electrical components (e.g., batteries, resistors, inductors, capacitors, switches, transistors) or a model of such an interconnection, consisting of electrical elements (e.g., voltage sour ...
that is monitored by signaling equipment. In normal operation, the
electric current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The movin ...
in the fence wires causes a
relay
A relay
Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts
An automotive-style miniature relay with the dust cover taken off
A relay is an electrically operated swit ...
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. The American Railway Engineering and Maintenance of Way Association /ref> 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
An electrical network is an interconnection of electrical components (e.g., batteries, resistors, inductors, capacitors, switches, transistors) or a model of such an interconnection, consisting of electrical elements (e.g., voltage sour ...
that is monitored by signaling equipment. In normal operation, the current through the plugs causes a
relay
A relay
Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts
An automotive-style miniature relay with the dust cover taken off
A relay is an electrically operated swit ...
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.
Structural fence
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.
Where used
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:
:1) On the
Union Pacific Railroad
The Union Pacific Railroad , legally Union Pacific Railroad Company and often called simply Union Pacific, is a freight-hauling railroad that operates 8,300 locomotives over routes in 23 U.S. states west of Chicago and New Orleans. Union Pac ...
's Moffat Subdivision, trains pass through three major canyons — Byers Canyon,
Gore Canyon
Gore Canyon, elevation , is a short isolated canyon on the upper Colorado River in southwestern Grand County, Colorado in the United States. Steep and rugged, the approximately 3 mile (5 km) long gorge was carved by the river as it pass ...
, and Glenwood Canyon, all three of which feature rock fences.
:2) Stretches of the BNSF Railway's Hi-Line, such as on the western approach to
Marias Pass
Marias Pass (elevation ) is a mountain pass in the Rocky Mountains in the western US state of Montana. Lying on the southern border of
Glacier National Park, it is traversed by US Highway 2 and by the BNSF Hi-Line Subdivision.
The pass is t ...
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
A track circuit is an electrical device used to prove the absence of a train on rail tracks to signallers and control relevant signals. An alternative to track circuits are axle counters.
Principles and operation
The basic principle behind t ...
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
The Pennsylvania Railroad (reporting mark PRR), legal name The Pennsylvania Railroad Company also known as the "Pennsy", was an American Class I railroad that was established in 1846 and headquartered in Philadelphia, Pennsylvania. It was named ...
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.
Alternatives
Several alternative technologies have been tried to solve the rock slide problem, including:
*Acoustic sensing
*