> There are later systems that still use track circuits, but provide more information to the driver and locomotive. These involve devices which sit between the rails and send signals to the train. Most of these are passive RFID-tag type devices. Some are wired into the signaling system.

Why do things this way, instead of just having the switches themselves notify control when they change state? (Or do they already?) That by itself tells you almost all you need to know about where a train is.