Verified answer

In modern signalling installations they carry the data of the trains identity. Every train has a unique 4 character code so that the signalman knows what it is, as the train moves from circuit to circuit the identity code travels with it.

In a few very modern signalling centres the identity code allows the signalling systems software to automatically route set for the trains (ARS) and in some cases make automated train announcements on stations, London Liverpool Street is an example.

They don't carry any data - its just an electric current that allows the signalling equipment to detect the presence of the train and thus control the signals and indicate the train's position to the signalman. The system will also detect broken rails and various obstructions, which will put the signal to the rear at red, and alert the signalling centre to a fault.

Communications between the train and signalling centre are by means of the 'Cab Secure Radio' system. This enables direct radio communication between the signalman and driver - and even the passengers via the PA system - as well as communicating various data about the train itsefl.

Basic track circuits don't carry any data about the train, other than the fact that it's there. More modern systems can carry full train identity and performance information. But just because the technology is capable of this it does not mean that it always happens.

Information is also exchanged between train and "shore" via satellite and terrestrial secure radio links and through various other types of transponder located track-side and between the rails.