I realize there is a specific case with this mu equipment where the car gets the emergency signal only via the pressure reduction in the train-line brakepipe, and that is with loose (not married-pair) trailer operation. Trailers were operated on all the L's, and on many subway lines, and their brake responses were usually governed only by the train line pressure.
In many mu operations, the sequence of the electrical signal was as follows:
Trip hammer hits the sensor and dumps the air on the first car.
The emergency application opens the line switch.
Opening the line switch on the first car transmits an electrical signal to open the line switch on all other motor cars.
All other motor cars then dump the air.
Not quite correct. The above was true of the much of the classic mu equipment that was ordered before WWII. All current equipment, from SMEEE control on, has electric sensing of main brake pipe pressure, and this information is transmitted instantly throughout the train so that all brakes in all mu cars act simultaneously and there is no time lag for pressure drop to travel through the length of the train. I believe this not only applies to New York's equipment but to all modern mu rail, rapid transit, and light rail equipment. Including Amtrak's original Metroliners.
In other words, there is reducancy: The informaiton is transmitted the length of the train both through reduction in train-air-line pressure and electric transmission.
There is also additional redundancy in the new automatic operation trains on the semi-isolated "L" 14-St.-Canarsie line. The trip hammers at the signals still provide additional safety control.
The emergency triggered by the Trip is transmitted pneumatically throughout the train via the Brake Pipe, Individual vent valves on each car help with the tranmission. Electric power is not involved.
A link to more information about ATS system on the NYCTA:
http://www.nycsubway.org/articles/signals_stops.html
Although officially frowned upon it is fun to seek out the railfan's window in the first car and watch the "T"'s fall in front of you....except on the elevated...yeah, ya see them, but there is a lot more to see, too! Many railfans underestimate the railfaning that can be seen and enjoyed riding mass transit such as the MTA, SEPTA, Metro, etc.. Heavier than light rail, sometimes lighter than heavy rail, but with all kinds of operations and hardware to be seen in actual operation! There really is a lot of railroading here!
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On the "regular" subway system there aren't any signs like that because it is understood that ALL signals have these trip arms. I really do not know of any exceptions.
When necessaruy to pass a red signal on the Transit System, the motorman, with conductor listening and approving, asks permission verbally of the desk dispather to "Key By." The desk dispatcher will then give him permission and make a written record of the situation. The motorman will either leave the train or if possible lean out of the side window and insert a key into the signal box. The action of this key will lower the trip arm.
It is also, of course, possibly for a signal maintainer to walk a line and dissable all trip arms in turn for some special movement. And then restore them to operation.
Yes, the automatic stop on both NYC's MTA and PATH. On PATH, you may see a sign with a "T" in a circle near a signal indicating that there is a trip device that will stop the train if the red signal is passed. I'm not sure if MTA has such signs. The trip device is an actual "T" shaped bar that will kick an$1****$2on the train's brake line to dump the air. You may (or used to be able to) actually watch these "T"'s fall back as the signal clears in front of an approaching train...used to be you could see out the front of PATH and MTA trains before full width vestibule cabs have been the standard.
Unlike the electrical and electronic automatic train stop that was pioneered by PRR in the 1920's and has been sophisticated and updated, the automatic train stop on the New York Subway system is mechanical. Whenever a signal is red, a pneumatically operated (possibly now electrical) shaft, normnally horizontal, rises at a point about 1/4 the distance from one rail to another, about 5 inches above rail height. This wacks a lever connected to the regular braking system of the lead car, "dumping the air" for an emergency stop. This loss of air pressure is sensed at the first car and the information transmitted electrically throughout the train to effect a quick but reasonably smooth emergency stop. Without the electrical sensing, it was introduced with the very first New York subway line in 1904.
It is possible to override and dissable its action within the train operator's controls, and that must have been done to permit the kind of operation you describe.
Was watching a Ken Burns PBS special about baseball. It mentioned the person who wrote the "Take me out to the ball game" song while riding the subway to the Polo Grounds (1900 - 1910).. At that tiime they showed a second subway train following a first by taking a picture from the second train. The spacing was only about 2 - 3 car lengths from rear of first to cab of second.
My questions
1. When did NYC TA put in their automatic train stop sysytem? (start to finish).
2. Was it put in because of rear end collisions that the PBS clip showed possible?
3. Any other comments?.
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