[kalleboo]

Perhaps they same way they do it on buses?

[superuser2]

No, you can't reliably receive GPS signals in a subway tunnel.

[nrb]

If you have wifi hotspots in every station you can track the trains that way, no GPS required.

[kalleboo]

I thought buses mostly used thr odometer? At least the ones where I lived used to use the odometer for the "next station" LED signs in the buses, and the bus stop arrival signs showed up not long after.