So, we don't know if this is the case, but one way to do this is not to ask the phones but the cells. The mobile network has to know where the phones are by cell; the cells are often small relative to the speed of the train; there are also cells installed specifically to improve service on trains, or provide a base station to the train wifi, or for communications to railway staff.
If you get a bunch of phones switching cell near simultaneously, you can tell that a train movement across the cell boundary has probably happened. Then correlate that with the other data feed about train blocks, and bob's your uncle.
If you get a bunch of phones switching cell near simultaneously, you can tell that a train movement across the cell boundary has probably happened. Then correlate that with the other data feed about train blocks, and bob's your uncle.
Only about 50% of trains have wifi: https://www.businesstravelnewseurope.com/Ground-Transport/UK... ; but it's easy to imagine getting the mobile hotspot on the train to share its GPS location as well.