SIGNALED CROSSINGS
By: Mr. Keith Wilson, Past railroad signal maintainer, Electrical engineering student

All signaled crossings share a basic component, a detector.  Although there are several different types of detection equipment, the general idea is to signal that a train is approaching a grade crossing by activating flashing lights and lowering gates.

In order for detection equipment to work a circuit is necessary. The detection circuit is the same in all circumstances, the rail. The first track circuit was patented in 1872 by Dr. William Robinson, who founded Union Electric Signal Company. The circuit works on the simple premise that when a train enters the energized area of track the axles cause an electrical short, thereby de- energizing a relay that operates the crossing lights and gates.  All circuits dealing with railroad signals and crossings operate on a normally energized fail safe. The circuit when operating correctly has power applied; when there isn’t power everything should be in the safest position possible, meaning crossing gates down and lights flashing. 

Some of the oldest grade crossings I maintained used a simple DC track circuit to fulfill this philosophy. Again this technology has been around since 1872. Maximum speed goes into determining the length of the circuit, and then the track is insulated at both ends. A battery is placed on one end of the circuit and a relay at the other end to operate the grade crossing.  The problem with these track circuits are that the crossings will operate as long as there is a train axle in the circuit. These installations also tend to be older, without gates with older lead acid batteries. This could cause a problem because the age of the batteries could affect their capacity.  When you lose backup power you lose all warning for the grade crossing.

In my opinion, one step up from the battery is an audio frequency overlay circuit. With this system, instead of a battery you have a transmitter, and at the other end you need a receiver to control the relay. This form of circuit has been used since 1903 and shares a negative with DC track circuit, in which they operate as long as there is an axle in the circuit.

To correct the problems with the previous systems, motion detectors are used. Instead of two or more locations, now they can have one location. These operate the crossing as soon as there is motion, and can recover the crossing as soon as they detect the train moving away from the crossing or no longer moving toward the crossing. A plus for the railroads with these systems is that they can park a train on the crossings circuit and it will recover or “time out” as soon as there isn’t any motion detected. If the circuit has been shorted by debris or other means you lose some of the approach that could cause the crossing not to activate quickly enough. Another problem with these circuits is that they can’t tell how fast a train is moving, so the crossing will operate longer with a slower train.

The most advanced system would be a predictor. Developed in the 1950’s by Stanford, it uses the same technology as the motion detector, but adds the ability to predict how long it will take the train to reach the crossing and will give the same amount of warning time no matter what speed the train is going. With these intelligent systems they also have added ability to control traffic lights and digital recording abilities. All of the systems described are simplified versions, depending on train speed and traffic on the highway and rail.   Grade crossings can get very complex using several overlapping systems.

All of the systems described have one common defect, the track. Anything that can short the circuit can have an effect on the safety of the grade crossing, including mud, debris and condition of the track itself. Mud can have an exponential effect on the signal received compared to the signal transmitted causing sporadic crossing activation. Debris can cause an even worse problem. In an advanced crossing the short can cause a shortening of the approach, causing the crossing not to give proper warning time. Change in temperature can also make the track expand and contract, breaking bonds and ruining insulated joints, again causing intermittent crossing failure.

(Disclaimer: The views, opinions, and subject matter are based upon Mr. Wilson’s personal experiences and are not those of the Foundation.  The Foundation in no way expresses expertise or common knowledge of its content).