The Westinghouse Interlocking Switch and Signal System
Scientific American—May 3, 1890


Some weeks ago we illustrated the electro-pneumatic block system of train signaling. In our present issue we present our readers with illustrations of some of the salient points of the Westinghouse interlocking switch and signal mechanism for use in train yards. It includes three operating agencies, electricity, pneumatic pressure, and hydraulic pressure. The work of throwing switches and of setting signals at safety is done by pneumatic pressure. The valves for regulating its action on signals are worked by electricity exactly as in the block system already described. The valves for regulating its action on switches are moved by hydraulic pressure.

Referring to the perspective view of the switch board, it will be seen to be a case upon whose front are two rows of handles. These handles, when moved by the operative, turn through an arc of a circle long vulcanite-covered spindles that run to the back of the case. These spindles are numbered in pairs of similar numbers, those to the left are rotated by the lower handles, those to the right by the upper handles. On the rear end of each spindle is a quadrant with locking detent, worked electrically. The upper row of handles operates the switches; the lower row operates the signals.

At the rear of each switch spindle, that is to say of every second one, is a three-way cock attached directly to the spindle, and therefore turned by the upper handle appertaining to the spindle in question. This cock is a part of the hydraulic system. Turned to the right, it operates by hydraulic pipe connections a valve in the neighborhood of a switch which may be a mile or more distant. The operation of this valve, which is connected to pneumatic pipes, admits compressed air to the actuating cylinder and piston, and throws the switch in one or the other direction. The switch-throwing mechanism will be described later on.

When a current of electricity is passed through the actuating magnet of the signal-moving mechanism, Fig. 3, which also may be at any distance, it opens a pneumatic valve, admitting air to the signal-actuating cylinder, placed on the semaphore post. The piston is forced outward and the semaphores are depressed to the safety position. This current is sent through the switch board, when the handles are set in proper position therefor. On each of the vulcanite-coated spindles are placed strips of platinum, and between the spindles are pairs of contact springs. These, with their connections, can be arranged in any way whatever to suit the conditions of the case. The circuit, including the semaphore magnet, is completed through one or more of these spindle connections. Hence the setting of any given semaphore at safety may be made to depend upon one or more switch movements, as necessary. After the switch handles in its series are properly set, they complete their part of the circuit by means of the platinum strips and springs. Then the final turn of the signal handle moves its spindle into position and the circuit is completed, and the semaphore descends to "safety." When the switches are to be changed, the signal circuit has first to be broken. This permits the air valve to close automatically, and the semaphore rises by the influence of its counterweights to "danger." The switches can then be moved as required. At the bottom of each signal-moving mechanism Fig. 3, is a circuit closer. This keeps a circuit closed as long as the semaphore is at danger. It is an independent circuit and works the locking of one of the quadrants on the rear of the signal handle spindles.

This, in connection with our former article, is enough to show how the signals are operated. The switch-throwing mechanism has now to be described. This is placed by the side of the track, near the switch. It is shown in Fig. 1. The hydraulic tubes, c c, connect with two cylinders, b b, and force the pistons one way or the other, according to the position of the three-way cock in rear of the switch board. At B is a reservoir, kept charged with compressed air by the regular air pipe, D. At E is a D-valve, exactly such as used in a steam engine, and which admits the compressed air into either one of the pipes, d d, and puts the other one of these pipes in communication with the open air. At A is a double cylinder with two pistons, connected by a rack and working a pinion. The partial section is shown in the upper left hand corner of the cut, in which K is the rack and A is the pinion. The air admitted by one of the pipes, d d, forces the pistons in one or the other direction, regulated by the D-valve, thus turning the pinion. As the pinion turns it carries around with it an arm attached to its spindle, a. In its revolution through about three-quarters of a circle the pinion has to successively perform the following operations: 1. To withdraw the locking bolt, H, from the hole in the locking bar, N; 2, throw the switch, by moving the rod, M; 3, return the locking bolt, H, to the other hole in the locking bar, N.

Referring to the sectional view, H is the locking bolt, N the locking bar, and M is the switch rod. If air is admitted to the right hand end of the cylinder, the other end communicating with the open air; the pistons will move to the left. The movement of the pinion may be divided into three phases, indicated by the dotted lines. Moving from 0 to 1 it withdraws the lock bolt, but practically does not move the switch bar, or at least only back and forth through the versed sine of the arc described by the crank pin between 0 and 1. From 1 to 2 the relations are changed; here the switch rod is moved through a longer distance, corresponding to the chord of the arc, 1-2, throwing the switch, while the lock bolt is only drawn back and pushed forward a trifle corresponding to the versed sine of the same arc, 1-2. From 2 to 3 the first relations are re-established; the lock bolt is thrust forward into the hole in the lock bar and the switch is hardly moved at all. A detector bar is worked simultaneously with the lock bolt by means of the rod, F, of the perspective drawing.

Referring again to the cut of the switch board, a series of studs or latches are seen projecting through holes below the switch handles. These, if moved upward, catch the lower projecting ends of the switch handles. They are raised and lowered by the agency of the signal handles. As each of these is moved it throws a notched bar that runs parallel to the face of the case to the right or left. The bar, according to the arrangement of the notches, throws upward or permits to drop any one or more of the latches. When raised a latch locks the lever above it, when depressed it frees it. Thus a mechanical interlocking of switch and signal handles is provided that by different disposition of notches may make the movements of one or more switch handles depend upon the movements of any given signal handle.

On the rear end of each signal spindle on the switch board is a quadrant which swings to right or left as the spindle is rotated. This has teeth in which a latch engages. This latch is operated by an electro-magnet in the rear included in the independent signal circuit just described. When the signal is at "safety," this circuit is open, and the magnet not attracting its armature, the latch, by gravity, locks the quadrant as to allow only a small degree of movement to the signal handle with its spindle. This movement is enough to close the main circuit and thus release the semaphore, which rises to "danger," but not enough to lower the interlocking latch or latches projecting from the front of the case, until the semaphore has risen the full distance to "danger." As it reaches this position the auxiliary or locking circuit is closed, the magnet attracts its armature, drawing the quadrant latch out of engagement with the quadrant, and the handle can now be swung clear over, unlocking the switch handles dependent upon it.

On the rear end of the switch spindle in the switch board are similar quadrants, whose locking latches are actuated by an electro-magnet connected with the circuit breaker on the switch-throwing mechanism. At the end of the locking bolt, H, of this mechanism, Fig. 1, is a circuit opener. When the bolt is in place and the switch locked as shown, the circuit is open. This opener is in circuit with the electro-magnet back of and actuating the quadrant-locking latch appertaining to its own spindle on the switch board. This is so constructed that a limited movement only is allowed when the circuit is open. This movement is enough to turn the three-way cock. The switch begins to move. As the locking bolt, H, Fig. 1, comes back, the circuit closes. The magnet attracting its armature unlocks the quadrant and instantly relocks it. This is done so quickly that the handle cannot be moved during the change. The switch continues its movement. As it is thrown and locked, the circuit opens, the quadrant is free, and the handle can be swung clear over. Its first movement was only sufficient to open the cocks, but not enough to close the signal-actuating circuit by the strips of metal on its spindle. This is done by the second motion. Hence as the signal cannot be moved until this circuit is closed, and as a semaphore cannot be set at safety without this closing of circuit, the protecting semaphore cannot be set at "safety" until all the switches in its system have been completely thrown and locked by the regular locking bolt.

Above the case containing the switch and signal handle mechanism and connections, and facing the operator, is a miniature model of the tracks and switches controlled by the switch board. The model has movable switches, so that at a glance the operator can see what position every switch in the system occupies. An annunciator drop is also placed in view of the operator. This is worked electrically by any train approaching the system, which train causes a bell to ring and also drops the shutter when it is within a mile of the track yard.

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