Earlier quoted context omitted.
One built in 1904. The control systems of the MTA are purely electromechanical. It goes way beyond what one would normally call a "legacy system". You can't record the position of trains because there is no way to snapshot the whole system. Only the signal tower local to that section of the system is aware of what's going on in its zone. This is a vestige of the design where there was no reasonable way to centralize…
So on the decentralized systems, how does one station know when to delay a currently-stopped train because there's a delayed train farther up the route? Do signal towers up ahead simply transmit to adjacent stations "Delayed train up ahead?"
It is fairly easy to hook up a "'low resistance between the rails aka 'wheels on the rails' aka 'there is a train here'" detector to a 'do not enter' signal a few hundred meters up, and to a 'drive slowly' signal some distance before that (http://en.wikipedia.org/wiki/Automatic_Block_Signal)