The ABC System - an early CTC


It was realized very early that the key to increasing the capacity of an American single-track railroad was the elimination of train orders, running trains by signal indication under a centralized control, which would supersede the superiority of trains. One way to do this was certainly known, the British manual block system. However, the expense for signalmen, block stations, interlocking, signals, and communications was prohibitive, except for those busy double-track lines that already had something better. In 1907, the Northern Pacific came up with a cheap method that only used existing facilities, operators and train order signals, to accomplish this objective.

In this system, called the "ABC" system, the operators at each end of a block and the train dispatcher cooperated to signal trains to stop or proceed by means of the block signal alone. Since the agreement of three minds was necessary to move trains, the system was regarded as safe enough that superiority of trains could be abolished.

All the operators, and the dispatcher, were on the same wire, so everyone was always aware of what was happening. When a train accepted a clear signal, it was also handed up a clearance card while moving at 25 mph or so, using a hoop. When trains were to meet, they were notified by orders on the card at the station in the rear that this was to occur at the next station. They would then approach this station with caution, taking siding or holding the main line as directed on the card. This was necessary, because the train order signals were not properly located to protect such a movement.

The system was first used near Spokane, Washington, and then was extended over some 600 miles of line as far east as Billings, Montana. The system is reported to have accelerated freight movement by 20 to 25 per cent, by eliminating the time required to prepare and deliver orders in the usual manner.

However, the system seems to have been somewhat half-baked. There were some collisions when freight trains did not agree on who was to hold the main track at meets. Passenger trains were seriously delayed, it seems, and this was cited as the main reason for abandoning the ABC System after a few years. By 1910 it was gone, replaced by the normal manual block, and by automatic signals between Billings and Livingston.

The ABC System must have been a severe strain on the dispatcher, since he could no longer think out problems and compose orders to solve them at one time, but had to react as the trains were moving, and make quick decisions. The operators were there as a check, but a good dispatcher does not depend on his operators for safety. These concerns could have been relieved by use of train staff, but this was never done. The small cost of the train staff was more than the zero cost of the ABC system, which doomed it. The ABC System was the first significant attempt to institute centralized control of trains in the United States over long distances.


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Composed by J. B. Calvert
Last revised 21 May 1999