Level crossing signals are the electronic warning devices for

road vehicle A vehicle (from la, vehiculum) is a machine that transports people or cargo. Vehicles include wagons, bicycles, motor vehicles (motorcycles, cars, trucks, buses, mobility scooters for disabled people), railed vehicles (trains, trams), water ...

s at

railroad Rail transport (also known as train transport) is a means of transport that transfers passengers and goods on wheeled vehicles running on rails, which are incorporated in tracks. In contrast to road transport, where the vehicles run on a prep ...

level crossing A level crossing is an intersection where a railway line crosses a road, path, or (in rare situations) airport runway, at the same level, as opposed to the railway line crossing over or under using an overpass or tunnel. The term a ...

s. The basic signal consists of flashing red lights, a

crossbuck A crossbuck is a traffic sign used to indicate a level railway crossing. It is composed of two slats of wood or metal of equal length, fastened together on a pole in a saltire formation (resembling the letter X). Crossbucks are sometimes suppl ...

and a bell (either a real bell or a speaker that mimics a bell sound), attached to a mast. At most crossings, the signals will activate about 30 seconds before the train arrives but there are sensors measuring speed so that the crossing knows when to activate; so, the slower the train is, the longer the delay and the faster the train is, the earlier the crossing activates. In some countries (such as the Czech Republic or Slovakia) there is also a flashing white light, which means that it is possible to pass the level crossing at a higher speed than generally allowed. At many crossings, there will be a

gate A gate or gateway is a point of entry to or from a space enclosed by walls. The word derived from old Norse "gat" meaning road or path; But other terms include ''yett and port''. The concept originally referred to the gap or hole in the wall ...

added to the signal. The gates will be fully lowered 15 to 20 seconds before the train arrives. The gates will rise and the signals will shut off once the end of the train clears the island circuit. The time interval may be controlled by a level crossing predictor, an electronic device which is connected to the rails of a railroad track, and activates the crossing's warning devices (lights, bells, gates, etc.) at a consistent interval prior to the arrival of a train at a level crossing.Marketing brochure by Westinghouse Rail Systems
(archive) Crossbucks also may have legends saying, for example: "RAIL ROAD CROSSING" (United States), "RAIL WAY CROSSING" (Australia and New Zealand), "PELIGRO FERROCARRIL" (Latin America, meaning DANGER RAILWAY), or be wordless (Canada and many other countries). Many states in the US are now requiring the use of this type of equipment at all newly constructed level crossings.

History

First devices

" Wigwag" was the nickname given to a type of crossing signals once common in North America, named for the

pendulum A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting, equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward th ...

-like motion it used to signal the approach of a train. Albert Hunt, a

mechanical engineer Mechanical may refer to: Machine * Machine (mechanical), a system of mechanisms that shape the actuator input to achieve a specific application of output forces and movement * Mechanical calculator, a device used to perform the basic operations of ...

Southern California Southern California (commonly shortened to SoCal) is a geographic and cultural region that generally comprises the southern portion of the U.S. state of California. It includes the Los Angeles metropolitan area, the second most populous urban ...

Pacific Electric The Pacific Electric Railway Company, nicknamed the Red Cars, was a privately owned Public transport, mass transit system in Southern California consisting of electrically powered streetcars, interurban cars, and buses and was the largest electr ...

(PE)

interurban streetcar The Interurban (or radial railway in Europe and Canada) is a type of electric railway, with streetcar-like electric self-propelled rail cars which run within and between cities or towns. They were very prevalent in North America between 1900 ...

railroad, invented it in 1909 for safer railroad level crossings. He utilized alternating

electromagnet An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in ...

s pulling on an

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...

armature. A red steel target disc, slightly less than two feet in diameter, serving as a pendulum was attached. A red light in the center of the target illuminated, and with each swing of the target a mechanical gong sounded. The new model, combining sight, motion and sound was dubbed the "Magnetic Flagman" and produced by the

Magnetic Signal Company The Magnetic Signal Company was an American company based in Los Angeles, California focused on railway signalling. The company was the manufacturer of the ubiquitous "Magnetic Flagman" wigwag railroad crossing (or level crossing) signal seen all ...

Modern devices

First developed in concept by the

Stanford Research Institute SRI International (SRI) is an American nonprofit scientific research institute and organization headquartered in Menlo Park, California. The trustees of Stanford University established SRI in 1946 as a center of innovation to support economic ...

in the late 1950s at the request of the Southern Pacific Company (the

Southern Pacific Railroad The Southern Pacific (or Espee from the railroad initials- SP) was an American Class I railroad network that existed from 1865 to 1996 and operated largely in the Western United States. The system was operated by various companies under the ...

, now merged into 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 Paci ...

), and patented in 1966,US Patent No. 3,246,143
/ref> the design goal of the level crossing predictor was to provide a consistent warning time for trains approaching a level crossing. Before this invention, the circuits used for activating a crossing's warning devices were very simple, activated whenever a train came within a fixed distance (hundreds or thousands of feet) of the crossing. This method required that the crossing be designed to accommodate a train approaching at the track speed limit, which leads to longer warning times for trains approaching the crossing at lower speeds. Very slow trains could have many minutes of warning time, thus delaying highway traffic unnecessarily.

Technology

All level crossing predictors rely on the changes in the electrical characteristics of the rails that occur as a train approaches the point at which the predictor is connected to the rails (the feedpoint). A railroad track occupied by a train or other electrical shunt can be viewed as a single-turn

inductor An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a c ...

shaped like a hairpin. As the train approaches the feedpoint, the area enclosed by the inductor diminishes, thus reducing the

inductance Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The flow of electric current creates a magnetic field around the conductor. The field strength depends on the magnitude of th ...

. This inductance can be measured by connecting a constant-current

alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...

source to the rails, and measuring the voltage which results. By

Ohm's Law Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equa ...

, the voltage measured will be proportional to the impedance. The absolute magnitude of this voltage and its rate of change can then be used to compute the amount of time remaining before the train arrives at the crossing, assuming it is running at a constant speed. The crossing's warning devices are activated when the computed time for the train to reach the crossing is equal to the programmed threshold time. The earliest level crossing predictors used

analog computer An analog computer or analogue computer is a type of computer that uses the continuous variation aspect of physical phenomena such as electrical, mechanical, or hydraulic quantities (''analog signals'') to model the problem being solved. In ...

s to perform this calculation, but modern equipment uses digital

microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circ ...

Implementation

A predictor includes a short "island" track which just covers the width of the level crossing. A predictor circuit in the middle of nowhere is usually terminated with a dead short across the rails at the outer ends. This assumes that there are no ordinary track circuits for block signalling purposes. Two predictor circuits may overlap, with tuned circuits used for one predictor to jump over the other. The tuned loops would be a dead short for one predictor, and an open circuit for the other.

References

External links

America on the move
on the National Museum of American History

on UP website {{DEFAULTSORT:Level crossing signals Level crossings Safety equipment