The Westinghouse Pneumatic Signal system, as before stated, is eentrolled by the location of the trains which are passing over' the road. It is illustrated in Fig. 9, and, as its name implies, the signals are brought to the clear position by the presence of compressed air in the cylinder. The magnet which controls the admission of air into the cylinder is directly controlled by the track relay, which is located on the signal post and is mentioned in the description of the rail circuit. A clear section permits the current from the track battery (see Fig. 5) to lass through the track relay, completing the circuit through the signal battery and energizing the magnet. This unseats the valve which is connected directly with the armature of the magnet, and permits the compressed air from the main pipe line to pass into the cylinder, thus driving out the piston, and lowering the signal to which it is directly connected. In actual practice the distant signal for a succeeding block is located on the same post with the home signal for the block immediately in advance. This arrange ment is for the purpose of indicating to trains a considerable distance in advance as to what condition the track is in, and permits of a much higher rate of speed than if trains received their signals only at the beginning of the block on which they wished to enter. The dis tant signal, however, may be located any desired distance from its home signal. In connec tion with the pneumatic block signaling system, a pneumatic lock is located at each switch connecting with the main track, which prevents the opening of a switch after a train has entered upon that section. and which, when the switch is once opened. sets all the signals controlling that section to danger. The compressed air which operates this system is derived from air-compressors located at any convenient point near the right of not to exceed 20 miles apart. As will be explained further on, this air can be and is used for operating the switches at interlocking points.
IxTramocsaxo. Tra6rrlord•ing.—The method of interlocking known as the Saxby & Farmer, and described in the previous issue, has been abandoned, and the Stevens type has now entirely taken its place. The Stevens locking has two forms. In the orig inal form, which is illustrated in Fig. 19. the tappet, which is directly connected with the lever, operates the locking bars, which run parallel with the greatest length of the machine, or, in other words, at right angles to the motion of the levers. This is object tionable from the fact that in large machines the locking bars become very long and heavy, and the method of driving them by the tappet creates a large amount of friction and results in considerable lost motion in time. In tile latest form, see Fig. 11, the Saxby & Farmer arrangement is retained, the flop of the Saxby & Farmer machine being replaced by a simple shaft connected with the link by a universal joint. A move me»t of the latch handle of the lever rotates this shaft and transfers the movement to the locking bar, which slides in a direction perpen dicular to the plane of the movement of the lever. By this arrangement the locking is made
extremely compact, and is located in plain view above the floor of the cabin, easy of access for cleaning and repairs.
The demands for more and cheaper interlocking have been met by the invention of several devices intended to combine the work of several levers into one. The most important of these is the selector, S, see Fig. 12, which is for the purpose of throwing several signals from the same lever. Theoretically, any number of signals, no two of which should be given at the same time, can be worked from the same lever, but in practice it is found best to limit this number to six or seven. The Selector is connected directly with the lever in the tower and also to the different switches, which, when they are in one position or the other, determine as to which signal can be thrown. The movement of a switch alternately connects or disconnects each of the rods leading to the different signals with the signal lever, but never connects more than one of these rods with the signal lever at the same time.
The Switch and Lock Movement for illustration, see Pneumatic Interlocking), which is now in general use in the United States and Canada, is a device for operating a switch, lock, and detector bar from the same lever. The original practice was to operate the switch by one lever, and the lock and detector bar from another lever, and it is still adopted in many cases. It is as expensive method, however, and is in many cases well replaced by the use of the switch and lock movement. In the operation of a switch through a switch and lock movement, the detector bar is first raised and the lock withdrawn ; immediately afterwards the switch begins to move, when, upon reaching its other position, it is again locked and the detector bar lowered. This sequence of movement is necessary in order to be certain that if a train were standing over a switch, the switch shall not be moved.
The Westinghouse Pneumatic interlocking Systrm is the application of compressed air for the operation of signals and switches which are electrically controlled from a central point. The appearance of the machine in the tower is shown in Fig. 13. The levers, which are at the top of the machine, and which all incline to the left, arc those used for operating the switches. The vertical levers, which are placed just to the right of and below the switch handles, are those which control the position of the signals.
A model of tracks is attached to the top of the machine, the switches on which receive their movement from the switch levers and which move in accordance with the position of the switch levers, showing at a glance the condition of the switches outside. Running through the machine parallel to its shortest axis are rollers formed of hard rubber, which, according to their position, make and break a contact through the different circuits. At the back of the machine are located a row of magnets con necting with each lever, called the indication magnets.