Systems of Automatic Train Control
MAGNETIC INDUCTION between the train and signal apparatus is used in the Strowger-
THE old saying, “familiarity breeds contempt”, might well be applied to the thousands of railway travellers who daily take their seats in the trains and are confident of a safe and speedy passage. Praiseworthy indeed are the untiring efforts of the railway engineers and staff who make it possible. Readers have already seen how Nature in her wildest moods can play havoc with a railway to the point of utter destruction; even when not totally destructive she can be capricious enough to dislocate train services and cause great delay and inconvenience. Probably everyone reading these words has made a train journey in a fog, which renders useless the finest system of visible signalling it is possible to have.
A blind man finds his way about by the senses of touch and hearing. An engine driver who cannot see beyond the chimney of his locomotive, and to whom, therefore, the wayside signals are invisible, has to rely on his ears. Thus in the early days it was soon realized that some sort of sound signal would have to take the place of visible signals in foggy weather. The well-
AN AUTOMATIC MACHINE which places a series of fog signals on the line. The detonators are exploded by the engine wheels and so give warning. The machine can be worked from an adjacent signal cabin or by a special “fogman” along the permanent way.
The “bang” portion of the modern version is a flat metal case about the size of a five-
Fogmen usually belong to the permanent way department, and are normally employed as platelayers, and so on. On some lines signalmen have to note a certain object some little distance from the signal-
Fog signalling is not a very pleasant job, as one may well imagine; but the companies do their best for the men, relieving them as often as possible, and providing fire-
It has long been maintained, however, that a system including audible or visible signals in the engine cab would he far preferable to absolute reliance on the human element, especially if this could be supplemented with some sort of mechanical control, such as that already applied to the Underground electric lines. Several such systems have been devised and tried out. Automatic train control is becoming quite common in America. In England, the Great Western Railway has a very satisfactory system operating on some of its main lines; the Southern Railway also has carried out trials, but of another system. In both systems the aspect of the signal is given by audible warning; and, if it is at “danger”, the brakes are applied automatically. In this connexion it is worth noting how the trend of opinion has completely veered round regarding mechanical control. At one time automatic signalling was considered inferior to human control, the fear being that something might go wrong and cause an accident. Experience proved the fallacy of this reasoning; we know now that it is the human being that is frail, while the mechanical signalman never tires and never makes a mistake. There is a parallel example on the roads, where the point duty policeman is gradually being superseded by “stop-
On the Great Western Railway, if the distant signal is “off” (clear) a bell rings in the engine cab. If it is “on” (warning or caution) a siren sounds and the brakes are partly applied; thus the engine driver knows exactly what aspect the signal is showing, even in the thickest fog. The indication is given about a quarter-
The apparatus is worked electrically by means of a ramp on the track and a contact shoe on the engine. The ramp consists of an inverted “T” bar of steel mounted on a baulk of timber and placed between the rails, so that the top is 3½ in above rail level. On straight lines the ramp is set slightly askew, being 1½ in off centre at either end; on curves it is placed 1½ in off centre, toward the outer rail. It is permanently fixed in position, and unlike electric fouling bars, treadles, point locks and other similar contrivances, does not move. The ramp can be electrified by current from a local battery, switched on through a relay operated by contacts on the lever in the signal-
The contact shoe on the engine is attached to the buffer beam, and has a vertical movement of 1 in, as it is adjusted to hang normally 2½ in above rail level. The ramp, being 3½ in high, lifts it the required amount. This lifting movement opens a switch. Inside the engine cab is a small case containing an electro-
The action of the apparatus is as follows. When running normally, the contact shoe on the engine hangs at its lowest position, and the switch attached to it is closed. Current then flows from one pole of the battery on the engine through the switch, and through the upper windings of the electro-
The signalman, having received the “all clear” from the next box ahead, pulls off his home and starting signals; this releases the interlocks on the “distant” signal lever, which he then pulls over. This action closes the switch on the lever; the distant signal arm drops to “clear” position and completes the circuits. Current now flows from the battery in the signal-
Along comes the train; the contact shoe on the engine strikes the ramp and is lifted by it, opening the switch attached to the shoe, and cutting off the current from the engine battery to the electro-
The driver, hearing the bell, knows that the signal is showing “clear”, even if the weather is so foggy that he cannot see beyond the chimney; while in clear weather it eliminates any possible chance of a signal being misread, or even missed altogether.
Supposing, however, that the section ahead is occupied, and the signalman leaves the signal in the “on” position. The ramp remains “dead”. When therefore the contact shoe on the engine strikes it and opens the shoe switch, and current from the engine battery is cut off from the electro-
The above system is giving perfect satisfaction and is in daily operation on many miles of the Great Western Railway. Its use is being rapidly extended and will eventually be applied to the whole of the line. It has so far been found necessary to give audible warning at distant signals only; but the apparatus could, of course, be applied to all signals if needed, whether these are manually or electrically operated.
There are four principal parts composing the apparatus on the engine: the receiver, the train control valve, the acknowledging plunger, and the hooter. The track equipment consists of two “inductors” at each signal. These are similar to very short ramps, and at distant signals are placed about 200 ft apart, and a little in advance of the signal. At “stop” signals they are combined as one unit. The first inductor is a powerful permanent magnet throwing an intense magnetic force at right angles to the rails, being placed crosswise between them. The second inductor is an electro-
THE HOOTER in the cab of an engine fitted with the Strowger-
The receiver on the front of the engine consists of four iron plates, mounted on a brass frame so that they can pass within four inches of the inductors; and connected to these plates are four pole pieces. Two of the plates are in line along the centre of the engine; the others are arranged one at either side. A centrally-
When the engine passes over the first inductor (permanent magnet) the two side plates on the receiver are energized, and the pole pieces connected to them immediately attract the adjacent arms of the “spider” armature, causing it to move round a little and open the pilot valve. This admits air to the underside of a diaphragm piston in the main control valve, which opens in turn and admits air to the train pipe, starting the brake application; and, as the air has to pass through the hooter, this at once begins to sound. This action always takes place at a distant signal, whether off or on. If the signal is showing “all clear”, the switch attached to it will be closed, the battery switched in, and the electro-
THE POSITION of the Strowger-
Supposing, however, that the distant signal remains at “danger”, it does not switch in the battery, so that the second inductor remains “dead”; consequently the hooter continues to blow, and the brakes will be fully applied unless the driver presses the acknowledging plunger knob, which returns the armature to its original position and closes the pilot valve. He then shuts off steam and applies the brakes with his own valve in the usual manner.
At a stop signal, the inductors being combined, no audible warning is given if the signal is off. The signal switch being closed, the electro-
As with the siren and bell device, trains can be operated with safety by means of this system of control in the thickest of fogs. Even in clear weather the audible warnings are a great aid to the engine-
(Right) SHOWING THE SHOE on the engine in contact with the ramp that automatically actuates the audible signalling apparatus in the cab.
(Below)THE WARNING BELL in the cab of a Great Western Railway engine. When the distant signal is clear this is indicated by the bell ringing in the engine cab. If the signal is at danger, a siren sounds and the brakes are partly applied by the automatic train control apparatus. The engine driver thus knows the aspect of the signal, whatever the weather.
TRACK EQUIPMENT in the Strowger-