AUTOMATIC TRAIN CONTROL Automatic train control is a term commonly used to describe a method of stopping or reducing the speed of a train without the intervention of the engineman, and, as generally employed, covers either an automatic train stop or a speed control system. In the first instance the brakes are applied until the train is brought to a stop; in the second, the brakes are applied if the speed of the train exceeds a certain designated limit, but may be released by the engineman after the speed has been reduced. These actions may take place at a designated point or at any point on the equipped territory depending on whether an intermittent or a continuous type is in use.
These devices provide means for transferring to the locomotive an indication of conditions existing on the track and furnish the connecting link between the track circuit and the air brake system of the train. The track circuit is really the block system or "train spacing" feature and the cab signals, train control, or roadside signals are merely the means of indicating to the engineman the conditions of the track as to occupancy by trains, open switches, broken rails, or some other obstruction.
There must be, therefore, apparatus, on the roadside to co-act with the locomotive apparatus, which may be a special adjunct, or the contact may be through the running rails. The locomotive must be provided not only with some device for receiving the in dication from the track by electrical means or by mechanical con tact, but also some method must be employed for exhausting the brake pipe air to atmosphere, and so causing the brake (or triple) valves on each car to function and apply the brakes. This is done either by a special valve opening directly to the atmosphere, or by apparatus which automatically operates the engineer's brake valve. Special appliances must be provided for the release of the brakes after the stop has been made. A "forestalling" device is also used except with speed control, which permits an alert engineman to prevent an automatic brake application.
the plain automatic trip, consists of a contact member, or trip, along the track which is brought into a position, when a stop is de sired, to make contact with a trigger or projecting arm on the locomotive which, when moved, will open the air brake pipe and cause the brakes to be set ; the trip may be set electrically or me chanically. This type is in use on elevated roads and subways. The ramp type has for its roadside element a length of iron of "L" or "T" section placed parallel to and outside the rail on the end of the ties and extending 5 or 6 inches above the top of the rail at its highest point in the centre of its length. Electric energy from a roadside source is supplied to it, controlled by con nections to the signal system. The engine carries a shoe adjusted to engage the ramp through a vertically moving member that, to secure proper electrical contact, is held down by air pressure or spring. As the engine circuits are opened every time a ramp is engaged, thus producing a potential stop condition, means must be provided for holding closed the engine circuit by energy collected from the ramp, if a stop is not intended. This is accomplished through electrical contacts, or switches, attached to the moving member of the shoe.
The devices just described are known as the intermittent con tact type, that is, physical contact is required between the track and engine elements, but to avoid this, the non-contact type has been developed. One form of this type uses permanent magnets located in the track between the rails, and associated with large coils which are designed, when carrying an electric current, to neutralize or deflect the magnetic field of the permanent magnets normally extending above the road bed to such a height that the engine apparatus will pass through it. During this passage, the lines of force either neutralize the lines of force of a small magnet carried on the engine or open contacts which control the electric circuits, thus initiating a brake application.