Railway slide fence
Encyclopedia
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.
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, 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.
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.
Railway signalling
Railway signalling is a system used to control railway traffic safely, essentially to prevent trains from colliding. Being guided by fixed rails, trains are uniquely susceptible to collision; furthermore, trains cannot stop quickly, and frequently operate at speeds that do not enable them to stop...
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
Railway signal
A signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state of the line ahead to train/engine drivers. The driver interprets the signal's indication and acts accordingly...
. As an alternative, a structural fence is designed to physically stop falling rocks from reaching the tracks.
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 fence 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
Electrical network
An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...
that is monitored by signaling equipment. In normal operation, the electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
in the fence wires causes a relay
Relay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...
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, 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
Electrical network
An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...
that is monitored by signaling equipment. In normal operation, the current through the plugs causes a relay
Relay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...
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:
- On the Union Pacific RailroadUnion Pacific RailroadThe Union Pacific Railroad , headquartered in Omaha, Nebraska, is the largest railroad network in the United States. James R. Young is president, CEO and Chairman....
's Moffat Subdivision, trains pass through three major canyons - Byers CanyonByers CanyonByers Canyon is a short gorge on the upper Colorado River in Grand County, Colorado in the United States. The canyon is approximately 8 miles long and is located in the headwaters region of the Colorado between Hot Sulphur Springs and Kremmling. U.S. Highway 40 passes through the canyon between...
, Gore CanyonGore CanyonGore Canyon is a short isolated canyon on the upper Colorado River in southwestern Grand County, Colorado in the United States. The steep and rugged canyon, approximately 3 miles long, was carved by the river as it passed the northern end of the Gore Range southwest of Kremmling...
, and Glenwood CanyonGlenwood CanyonGlenwood Canyon is a rugged scenic canyon on the Colorado River in western Colorado in the United States. Its canyon walls climb as high as above the Colorado River. It is the largest such canyon on the Upper Colorado...
, all three of which feature rock fences. - Stretches of the BNSF RailwayBNSF RailwayThe BNSF Railway is a wholly owned subsidiary of Berkshire Hathaway Inc., and is headquartered in Fort Worth, Texas. It is one of seven North American Class I railroads and the second largest freight railroad network in North America, second only to the Union Pacific Railroad, its primary...
's Hi-LineHi-LineThe Hi-Line is a geographical term referring to the portion of the northern United States south of the Canadian border along which runs the main line of the BNSF Railway and U.S. Highway 2...
, such as on the western approach to Marias PassMarias PassMarias Pass is a high mountain pass near Glacier National Park in northwestern Montana in the United States.The pass traverses the continental divide in the Lewis Range, along the boundary between the Lewis and Clark National Forest and the Flathead National Forest...
, and a small canyon called Bad Rock Canyon east of Columbia Falls, MontanaColumbia Falls, MontanaColumbia Falls is a city in Flathead County, Montana, United States. The population was 3,645 at the 2000 census.-Geography:Columbia Falls is located at ....
, are equipped with rock fences.
Consequences of slide fence activation
Train dispatcher
A train dispatcher is employed by a railroad to direct and facilitate the movement of trains over an assigned territory, which is usually part, or all, of a railroad operating division. In Canada the train dispatcher is known as the rail traffic controller...
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
Track circuit
A track circuit is a simple electrical device used to detect the absence of a train on rail tracks, used to inform signallers and control relevant signals.- Principles and operation :...
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
Pennsylvania Railroad
The Pennsylvania Railroad was an American Class I railroad, founded in 1846. Commonly referred to as the "Pennsy", the PRR was headquartered in Philadelphia, Pennsylvania....
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
- Electromagnetic sensing
- Seismic sensing
- Visual sensing, using cameras