MBTA F40PH-2C 1050 wrote:How long has the B&A's signal system been like this and why is it like this? save power?
Since the late 1980s, I believe. The system is one of cab signals w/o wayside intermediate signals. This was an experimental Conrail project designed to eliminate the maintenance of the intermediate signals. It also adds to safety in that operators of trains have a continuous indication, displayed in the cab, of the condition of the track ahead.
The track between controlled points is divided up into segments. Cab signal indications change according to whether these segments in front of a train are clear of trains and other equipment that shunts track circuits. Open switches on the main track also affect the signals displayed in the cab -- as do broken rails or loose bond wires.
A train approaching a CP which has it's signal at "stop," (or an occupied segment of track) will first get an "approach" in the cab, and then a "restricting" as the train gets closer to the wayside CP signal (or occupied segment).