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.

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.

Few questions, wonder if anyone knows the answers:

1. So it's Trainline on a persons phone that is tracking this info and using it to enrich this service? I use Trainline and didn't know it was doing that, but I do have location permissions on because I was told that powered the search picker when I started using the app.

2. What did they use _before_ Trainline? Or was Trainline selling user location data to them?