Well, that's one of the most archaic systems out there: http://gothamist.com/2015/07/28/subway_steampunk_video.php http://gothamist.com/2015/07/28/subway_steamp

Well, that's one of the most archaic systems out there: http://gothamist.com/2015/07/28/subway_steampunk_video.php 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."