https://en.m.wikipedia.org/wiki/Comm..._train_control

This is what the network I'm working on in Bangkok uses. Along with automatic train control.

I said GPS-esc positioning system. GPS units are actually the back up on this network as it can't be relied on. What is used is a heap of passive RFID tags and using the very accurate tachometer.

You can also use a leaky cable and receiver to better know the train location and close the gap between trains.