It seems that often the signalling system and the real time location reporting system aren't integrated. In NYC on many lines you get reports like 'there's a Brooklyn-bound train 2 stops away' that they make over the loud-speakers. Makes me wonder whether they have a separate device that just picks up when trains pass by a track segment, rather than using the information available via the signalling.
In New York the signalling system on most lines is ancient. The L train has a CBTC signalling system and it has real time arrival information. The 1/2/3/ and 4/5/6 have had upgrades, though not to CBTC, to provide similar countdown clock functionality. The 7 train is undergoing work to install CBTC.
On other lines, my understanding is that the signalling system doesn't know which train occupies a block, just that a train occupies the block. Which block is occupied isn't sent to a central control center either.
So "there's a Brooklyn-bound train 2 stops away" is the best it can do, since it can know based on which side of the tracks the train is on where the train is going. It can't know the letter or number.
Well, that's one of the most archaic systems out there: http://gothamist.com/2015/07/28/subway_steampunk_video.php (Not the oldest, but using the oldest technology). If I understand it correctly, the only information available from the signalling system there is the rudimentary "there's something on this track segment now; it's up to the human controller to figure out what it is, where it came from and where it's headed."
