From what I once read, only the Red & Orange Lines have such systems presently in place.
The Green and Blue Lines are still using the old "traffic light" system, which has come under tremendous fire ever since that tragic & fatal crash that had occurred on the Green Line in May, '08 where the driver of the 2nd trolley was killed and scores of people were injured.
Not quite. Here's a layman's breakdown of the different signal systems.
- Green is totally operator-controlled with no enforcement for speeds or stops. Pure "traffic light" system where each signal block is marked by lights the operator has to look at trackside.
- Blue is NYC Subway-style. It has the same wayside traffic lights the operator has to obey. But there's an enforcement mechanism in the form of trackside-mounted mechanical trips next to each traffic light signal. When the trip arm is in the down position, the signal permits totally free movement. When the trip arm is up, it'll strike a target underneath the train and make the train immediately stop. Between signals the operator has the same leeway he or she would have on the Green Line.
The trips can only enforce absolute stops. But they are jury-rigged for *crude* speed enforcement by switching from up to down one after the other (like falling dominoes) timed to the speed limit the train is supposed to be following. Go too fast from one signal block to the next, and the train will hit the next trip before it's moved into the down position and will get a stop penalty. The operator can still floor it between blocks, but won't be able to blow into the next block without getting penalized. This eliminates 99% of the train-on-train collision risk from operator error because--barring catastrophic signal failure--the signal system is never going to allow 2 trains into the same block. The trailing train will always be stopped dead at a trip.
Downside of trips is that it's extremely hardware-intensive. Every traffic light needs those mechanical arms mounted trackside. Above-ground every pair of trip arms needs an electricity-sucking heater so it doesn't freeze. The arms, because they physically strike the train, break off all the time and cause signal-related delays when one faults. They have to be inspected and replaced frequently. Red retired this system on the subway and Ashmont Branch in 1988 (Braintree was built modern from Day 1). Orange retired it north of Haymarket in 2004 (SW Corridor was built modern from Day 1). Both lines got better overall reliability from simplifying their trackside hardware. Even though Blue has the least traffic need for a next-gen signal system, the T has the most potential money to save long-term by doing Blue first. Especially when shoreline flooding is taken into account. NYC Subway suffered such extreme damage from Hurricane Sandy in large part because so many miles of the same electrical/mechanical signal hardware got fried. They're accelerating the pace of their CBTC signal rollout in part because of the Sandy relief aid they got.
- Red and Orange use ATO. That's an pulsing electromagnetic charge that goes through the running rail and gets picked up by the train. The pulses are pumped out by little circuit boxes every several dozen feet with wires welded to the rails (you can sometimes see them inside the track pit at stations), a significant reduction in the amount of trackside hardware vs. trip arms and traffic lights. The pulses come in 1-bit bandwidth--meaning they just turn on and off without conveying any other information--and the pattern of pulsing corresponds to a "code" that the train interprets. Basically, the train sees a signal through the pattern of intermittent "flicker" it picks up from the pulses. The pulse codes correspond to speed limits on the signal block and what type of signal--clear, restricting, etc.--it is.
ATO works on the same general principles as the Blue Line's system, only all of the trackside moving parts are replaced. The operator sees the traffic lights on his or her control stand, not by the tracks, and the mechanical trips that enforce absolute stops are replaced by pulses interpreted as stop codes. The operator still has to manually follow the speed limit. The difference with ATO is that these pulses can prevent speeding between blocks, whereas on the Blue Line the trips will only catch Larry Leadfoot at a signal. An overspeeding Red or Orange operator will hear an alarm buzzer and have 3-5 seconds to get back within the speed limit, or the ATO will "trip" them into a full stop. Sort of like a Blue Line that has many, many more trip arms spread throughout.
- CBTC is way more sophisticated than ATO. ATO, like all the other more primitive systems, is strictly 1-way communication: the signal system tells the operator what to do and enforces a penalty on the train if the operator doesn't do what he/she is supposed to. ATO is also intermittent; it takes some degree of lag time for it to read the "flicker" of electromagnetic pulses and interpret what the signal code is. So it has to be laid out the same way as the traffic lights and mechanical trips: in rigid, permanent signal blocks. That's the best you can do with any sort of 1-way communication. Even the ATO pulses are just "on/off, on/off, on/off" commands given very fast, so it doesn't do much more than barking out an action-reaction set of commands.
CBTC is both 2-way and continuous. The train and the signal system can talk back-and-forth to each other in real time using encrypted wireless or RFID signals. There'd be transponders wired up to fiber optic cable along the track, and they'd ping a receiver on the train to establish the continuous computer link. That's even less trackside hardware than ATO, since you only need as many transponders to maintain a short-distance wireless signal at fail-safe reception. CBTC lets signal system and train adjust to each other in real time, at high bandwidth. That 2-way communication and extra bandwidth is what opens up all the power of computers to help fine-tune schedule management. The signal system can manage the schedule based on what every train on the line is doing, because it's talking to all of them at once and can make decisions based on talking to all of them at once. You can even program CBTC to eliminate fixed signal blocks altogether (known as "moving block"); the signal system becomes entirely 'virtual' based on positioning of one train vs. the next instead of at fixed intervals...allowing closer or more flexible spacing and some degree of "self-healing" for train bunching.
It automates more of the operator's functions by continuously managing speeds and stops, closing some of the last loopholes where the operator has even momentary chance of overspeeding. Indeed, CBTC is how most modern driverless trains are set up. That wouldn't happen here--there's still lots of duties the operator has to oversee onboard a legacy subway system's trains--but the technology does allow it. Because it's "software" more than "hardware", you can program it to be almost anything you want it to be.
