There are a number of IR detectors available commercially specifically intended to indicate when a train passes them. With the number that we wanted this would have been quite an expemsive solution. So we tried our own method using tiny “Hall probes” that are easily small enough to fit between the sleepers and are cheap.

Each probe was connected to a separate digital input on an Arduino mega – which has lots of input pins. The main controller an then talk to this Arduino to knows when a train passes over a Hall probe. Also which Hall probe it is. 

To make the Hall probes work a very tiny magnet is attached to the underside of the train. It is small enough not to interfere with the way the train works.

We made a number of probes each of which looks like the image below on the left. The Hall probe signal is  analog and it is much much easier to process if it is converted to a digital signal. This was done with comparators which create a digital output when the analog voltage from the probe exceeds a certain value. Each comparators can handle four inputs and the circuit board to do this is shown on the right.