# A curiosity question (re freight train with many upfront pulling engines)



## Mist (Sep 6, 2019)

I have often wondered, when I see a freight train with many upfront pulling engines (two or more; sometimes up to five) how those engines are coordinated by the engineer in the lead engine? Are they somehow wired together, or via radio waves, or? to have them coordinate their pulling movements together? Taking it a step further, when I see a pusher engine or two way back on the train in the middle of a stream of cars, or pushing from the very rear of the train; again, how are they being controlled by the engineer? I assume those pushers are unstaffed? Have there been any problems where a pusher engine a mile away from the lead engine doesn't 'follow orders'?


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## ehbowen (Sep 7, 2019)

For head-of-train units, there is "multiple unit" control which was developed by Electro-Motive Corporation (which later became part of General Motors, EMD). It uses a 27-conductor cable which is connected between locomotives and carries information such as the eight throttle positions (plus idle and shutdown), reverser direction, dynamic brake control, sander signal, etc. In addition there are additional air hoses which are connected to allow all compressors to work together, as well as the independent brakes on all of the locomotives. This system was a major innovation...in steam days each locomotive required its own fireman and engineer when running double-headed, and the only way to coordinate actions was with whistle signals...and is one major reason why Diesel supplanted steam so quickly and decisively.

Other manufacturers designed their Diesel locomotives to be compatible with EMD's system, and it became the de facto standard. For 'push-pull' trains which operate with the locomotives at one end and a 'cab car' at the other, all cars in the train have to be equipped with that 27-conductor cable in order to provide control continuity. If you check private varnish listings (or ask @Seaboard92) you'll see that many Amtrak-certified private cars have been retrofitted with such a cable so that they can run in one of these trains.

For long freight trains with mid-train helper engines, there is normally a radio remote control system...I believe that Locotrol is one such trade name. While I'm not familiar with its inner workings, the only safe behavior I can imagine is for it to drop into idle if communication with the master controller in the lead locomotive is lost...as when passing through a tunnel or similar.


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## Mist (Sep 7, 2019)

Thanks so much! This info is very helpful in understanding the procedure.


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## Seaboard92 (Sep 8, 2019)

I think a lot of the PVs actually had the pass thru MU installed by commuter railroads, or Amtrak in their previous life. There are still several cars that lack it.


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