How does a train make the railroad crossing signals operate? — Michael, East Rockaway, N.Y.
Marshall Brain Answered:
Here is your classic train crossing signal. As the train approaches, the gates come down, the lights flash and the bells ring:
How does the crossing signal know that the train is coming? There are lots of ways that the sensing of the train could be accomplished. You could have a weight sensor or a big push-button switch in the track. You could use light beams or ultrasound. to detect the train. You could use induction loops like they do with cars at traffic lights.
But the most common method is to use the fact that the train’s wheels are made of steel, and these steel wheels are connected together with a solid steel axle (see this page to understand how trains can round corners even though they have solid axles). The train essentially becomes part of a giant switch, with the wheels/axles completing a circuit across the two rails. One rail carries voltage, one is ground, and the train completes the circuit between them. The completion of the circuit activates the signal. These two pages offer more detail: