Motion Sensing: How does that work? Motion sensing for grade crossings isn't done with motion sensors like in a security system. Instead, specialized track circuits are used. The track circuits use a frequency to exploit the electrical properties of the steel rails. As a train approaches the road, the length of rail between the road and the train decreases and the electrical current changes as a result. Sophisticated circuits detect the change and recognize it as a train approaching.
Constant Warning Crossings: What are they? Constant warning crossings utilize more sophisticated motion sensor technology that takes the analysis a step further. The system detects the changing electrical signal as a train approaches and also determines its distance from the road and the speed at which it is traveling. Knowing this information allows the control circuits to provide motorists with constant warning time. A fast train will cause the crossing to start operating while the train is far and a slow train will cause the crossing to start when very close to the crossing.