Automatic interlocking protects crossing of two main lines
Replacing mechanical with automatic plants reduces delays and saves 30 per cent on investment.
In October, 1933The derailment occurred on October 9, 1933The Toronto Daily Star, Tuesday, October 10, 1933, p. 24, c. 8., a Canadian National train on the double-track Toronto-Port Huron main line was derailed at the crossing of the single-track Toronto-Detroit line of the Canadian Pacific at Komoka, Ont., and destroyed the tower and interlocking machine of the mechanical plant. Instead of replacing these facilities, it was decided to install an automatic plant. As these are both main lines, the traffic is rather heavy, involving at present a total of about 50 trains over this crossing daily, of which the Canadian Pacific operates 7 passenger trains and about 12 freight trains, and the Canadian National operates 8 passenger trains and about 22 freight trains. With this volume of traffic, the practicality of an automatic interlocking might be questioned, but operating officers of both roads report that there have been fewer train detentions at this crossing since the automatic plant was installed than were experienced with the mechanical plant. An important consideration is that, with the mechanical plant, the towerman was frequently unable to change the line-up fast enough to give the approaching train a clear approach signal. As a result, the faster trains were frequently required to reduce speed and some times to stop. With the automatic plant, the line-up changes immediately after a train clears the home signal limits, and there is no delay to an approaching train to obtain a clear approach signal. The operation of the automatic plant has, therefore, been highly satisfactory and, incidentally, has dispensed with the services of the three operators and relief men. The installation cost $14,600 and effects an annual saving in operating expenses of $4,416, which represents a return of 30 per cent on the investment.
Layout of plant
Track and signal plan of automatic plant
As shown on the track diagram, the home signals are from 560 ft. to 587 ft. from the crossing, and the approach signals are 2,500 ft. in advance of the home signals. The approach control sections range from 2,500 ft. to 3,200 ft. depending upon grade conditions. Providing the plant and approach sections are unoccupied, a train on either road, which enters its approach section, causes the signals to clear for its route, thus locking the circuits to prevent opposing signals on the other road from being cleared.
The signals
Home signal on the Canadian National
The signals are the searchlight type, the home signals operating to two aspects, red for stop, and green for clear. On the Canadian Pacific each home signal is equipped with an inoperative lower unit showing red, thus completing the standard C.P.R. aspects for an interlocked home signal. On the Canadian National the two high signals for normal direction operation are each equipped with a marker displaying lunar white, thus forming the standard C.N.R. aspect for a home signal at this type of interlocking. The reverse running signals on the Canadian National are dwarfs, and as such are standard interlocking signals. All signals are lighted on approach control with the exception of the approach signals on the C.N.R., which burn continuously so as to serve as location markers, so that enginemen of trains running against the current of traffic may know they are approaching the plant [because there are no reverse running approach signals].
In case the signals should fail to clear, which has not happened since the plant has been in service, a train is stopped short of its home signal, following which the conductor or trainman proceeds to the crossing and unlocks a small box. He then observes a set of indicator lamps to see whether any trains are approaching on the other road, and, if not, he opens a knife switch, which cuts off energy from all of the home signals and holds them in the stop position. The train is then flagged over the crossing and the knife switch returned to normal before the train proceeds.
Flexible control
From the plan, it will be noted that a passing track switch is located in the westward approach section in the Canadian Pacific. An arrangement including a time-element relay is provided such that if a Canadian Pacific train enters this approach section and occupies it for more than two minutes without passing the approach signal, the C.P.R. home signal will resume the normal indication, leaving the crossing free for a C.N.R. train if one should approach. The C.P.R. westbound home signal will again clear when the train passes the approach signal.
In general, the control circuits for this automatic interlocking following A.R.A. Signal Section standards for such plants, which are based on the stick-relay system. For example, when a Canadian Pacific home signal is cleared by an approaching track, the stick relay for the Canadian Pacific is released and prevents any signal on the C.N.R. from being cleared until the Canadian Pacific train has cleared the home signal territory.
This automatic interlocking was planned and installed by the signal department forces of the Canadian Pacific, the principal items of signal equipment being furnished by the Union Switch & Signal Company.