I genuinely don’t know. Does the bus or the train or whatever report its position back to the tracking system using a data connection over the mobile phone systems, does it use the GPS satellites, or does it do something else altogether? It’s one thing if the train knows where it is, it’s quite another to share that knowledge with the rest of the world.
I'm coming late to this bit of the discussion!
There are two separate issues, GPS tracking and signalling which have become confused.
A conventional signalling system can locate a train only to the accuracy given by the length of the track circuits or axle counter blocks - that is to within a kilometer or less depending on the length of the individual block. In absolute block areas with long blocks then this distance could be several kilometers.
The GPS radio locks on to the base station with the strongest signal and the base stations will hand off the mobile from one base station to the next as the signal strength waxes and wanes as the vehicle moves. This is standard GSM technology. There is no inherent position tracking in GSM - a mobile can be located, in best case to within a few metres, by triangulating the signal strength received at several base stations but this is a process carried out by the network and not the mobile. In an emergency the all trains stop signal is received by the base station that the mobile is talking to and re-radiated. This is why trains on parallel routes can also be stopped as the base stations cover areas which may include other lines.
The data transmission part of GSM-R is intended to carry the messages generated by the European Train Control System part of ERTMS - these report the beacon (balise) that the train has passed to the signalling centre and receives the movement authority in return. GSM-R does not itself track the position of the train - it carries the signalling system messages and it's the signalling system which knows where the train is to within a few metres as well as the vehicle's speed and direction and the track it is on.
Vehicle tracking using GPS, or the European Galileo or the Russian GLONASS equivalents, uses the GPS receiver (which these days are very small - there's one in every smartphone!) in the vehicle. This reports its position (and speed and direction and altitude if needed) at intervals using the standard mobile phone network. As it's data being transmitted, rather than voice, the mobile transmitters often use whichever mobile network offers the strongest signal at that location. Depending on the level of encryption to which the user is entitled positional accuracies to the centimeter are possible. GPS tracking is not at the moment used for signalling purposes, at least on European railways, as in times of crisis the accuracies can be degraded or the whole network turned off. It is however used for other purposes which are not safety critical - measuring dwell times for example.