Usually, all railway lines must know the position of every train at any time. This is simply a requirement for avoiding crashes. As far as I know, there are usu

Usually, all railway lines must know the position of every train at any time. This is simply a requirement for avoiding crashes. As far as I know, there are usually pressure/induction sensors at regular intervals that detect train positions. Maybe that is what they are using? Electric railways usually only supply actually used sections of the railway with power, so they have to know train positions, too.