A
grade crossing predictor is 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 grade crossing.
First developed in concept by Stanford Research in the late 1950s at the request of the Southern Pacific Company (the
Southern Pacific RailroadThe Southern Pacific Transportation Company , earlier Southern Pacific Railroad and Southern Pacific Company , and usually simply called the Southern Pacific, was an American railroad. The railroad was founded as a land holding company in 1865, later acquiring the Central Pacific Railroad by lease...
, now merged into the
Union Pacific RailroadThe Union Pacific Railroad , headquartered in Omaha, Nebraska, is the largest and oldest operating railroad network in the United States. James R. Young is president, CEO and Chairman....
), the design goal of the grade crossing predictor was to provide a consistent warning time for trains approaching a grade crossing.
Previously, the circuits used for activating a crossing's warning devices were very simple, and activated them whenever a train came within a fixed distance (hundreds or thousands of feet) of the crossing.
A
grade crossing predictor is 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 grade crossing.
History
First developed in concept by Stanford Research in the late 1950s at the request of the Southern Pacific Company (the
Southern Pacific RailroadThe Southern Pacific Transportation Company , earlier Southern Pacific Railroad and Southern Pacific Company , and usually simply called the Southern Pacific, was an American railroad. The railroad was founded as a land holding company in 1865, later acquiring the Central Pacific Railroad by lease...
, now merged into the
Union Pacific RailroadThe Union Pacific Railroad , headquartered in Omaha, Nebraska, is the largest and oldest operating railroad network in the United States. James R. Young is president, CEO and Chairman....
), the design goal of the grade crossing predictor was to provide a consistent warning time for trains approaching a grade crossing.
Previously, the circuits used for activating a crossing's warning devices were very simple, and activated them 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. Unfortunately, this made for longer warning times for all trains approaching the crossing at lower speeds. Very slow (5
mphThe mile per hour is a unit of speed, measured in Imperial units expressing the number of international miles covered per hour.It is currently the unit used for speed limits, and speeds, on roads in the United Kingdom and United States...
, for example) trains could have many minutes of warning time, thus delaying highway traffic unnecessarily.
Technology
All grade crossing predictors rely on the changes in the electrical characteristics of the rails which occur as a train approaches the point at which the predictor is connected to the rails (the feedpoint). Essentially, railroad track occupied by a train or other electrical
shuntShunt may refer to:* Shunt - a hole or passage allowing fluid to move from one part of the body to another* Shunt - a device allowing electrical current to pass around a point in a circuit...
can be viewed as a single-turn
inductorAn inductor or a reactor is a passive electrical component that can store energy in a magnetic field created by the electric current passing through it. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
shaped like a hairpin. As the train approaches the feedpoint, the area enclosed by the inductor diminishes, thus reducing the
inductanceInductance is the property in an electrical circuit where a change in the electric current through that circuit induces an electromotive force that opposes the change in current ....
.
This inductance can be measured by connecting a constant current a.c. (
alternating currentIn alternating current the movement of electric charge periodically reverses direction. An electric charge would for instance move forward, then backward, then forward, then backward, over and over again...
) source to the rails, and measuring the voltage which results.
Ohm's LawIn electrical circuits, Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them....
tells us that with a constant current source, the voltage measured will be proportional to the impedance. The absolute magnitude of this voltage and the time-rate-of-change of that magnitude can then be used to compute the amount of time remaining before the train arrives at the crossing. (For a constant-speed train only.)
The crossing's warning devices are activated whenever the computed amount of time remaining before the train's arrival at the crossing is less than or equal to the desired (programmed) amount of warning time. However, each equipment manufacturer has their own special algorithm for determining this. The earliest grade crossing predictors used analog computers to perform this calculation, but modern equipment uses digital
microprocessorA microprocessor incorporates most or all of the functions of a central processing unit on a single integrated circuit . The first microprocessors emerged in the early 1970s and were used for electronic calculators, using binary-coded decimal arithmetic on 4-bit words...
s to do this.
Legal
Many states in the US are now requiring the use of this type of equipment at all newly-constructed grade crossings.