of the rocking-bar is sufficiently below the top of the rail to be well clear of the flange of any passing wheel; but while changing from the one to the other position, and when the pivoted arms are vertical, or at half-stroke, the upper surface of the rocking-bar is about level with the top of the rail, and right in the pathway of the wheel-flange. It is evident, therefore, that when the pivoted arms are set in the forward or backward position, and one of the wheels of a train or vehicle has passed on to the rail over the rocking-bar, the latter cannot be changed or raised and pulled over to the opposite extremity so long as any one of the wheels of the train or vehicles remain over the rocking-bar.
As the same ground-crank which pulls over the pivoted arms from backward to forward also withdraws the locking-bolt B, the latter is thus held securely in the hole or slot of the transverse rod, E, until all the wheels of the train have passed off the rocking-bar. The operation of changing the points from one road to another is very simple. By means of the rodding G, worked by a lever in the signal-cabin, the locking-bolt B is first withdrawn from the slot; the points are then pulled over into the reverse position by the rodding H, and the locking-bolt B is again set back into one of the slots by the rodding G. Sometimes, for economy, the points, bolt-lock, and rocking-bar, are all three worked by one lever in the signal-cabin, and one set of rodding on the ground, as shown in [Fig. 482]; but the arrangement is neither so perfect nor so secure as that shown in [Fig. 480]. Where there are two sets of rodding and gearing, the failure or breaking of either of them prevents the complete combination being made, and indicates at once to the signalman that something is wrong; but when there is only one set of rodding a breakage may occur without giving any tangible evidence to the signalman of the defect, and he may proceed to pull over his signal lever in ignorance that the points have not been properly made and bolted. To avoid an accident taking place from the failure of either rodding or gearing, the signal-detector has been devised, so as to prevent the possibility of pulling over the signal wire until the points and locking-bar are both in their proper positions.
The signal-detector is applied in several forms; the one shown in [Figs. 480 and 483] will explain the principle on which its efficacy depends. A transverse rod, I, attached to the sliding-rail,
extends out beyond the rails, and is formed into a flat bar or plate, J, sliding through the guide-holes K, K in the casting L. Short upright levers, M and N, work on trunnions fixed in the casting, and to M and N are attached the wires leading from the signal-cabin and continuing on to the signal-posts, as shown in elevation in [Fig. 483]. Two slots are cut in the plate J to receive the curved arms of the levers M and N when they are drawn downwards to pull off the corresponding signals. Neither of the levers, M or N, can be drawn over unless there is a slot immediately under the curved arm into which it can enter. When there is solid plate under a curved arm, the short lever cannot be pulled over, and the signal therefore remains at danger. The slots in the plate J are spaced so that one will be brought into position for one of the curved arms, when the points are close home for the main line, and the other slot for the other curved arm, when the points are set for the branch line or siding. The two slots cannot be under the two curved arms at one and the same time, as one of the signals corresponds to the main line and the other to the branch line or siding.
In some forms of signal-detector the transverse rod I is joined on to a vertical bar which slides through guide-holes in a casting something similar to the arrangement shown in the casting L. Longitudinal guide-holes, parallel to the line of rails, are made in the casting a little above the transverse rod-bar, and through the longitudinal guide-holes slide two vertical bars which are attached to, and form part of, the wire connections to the two signals. The wire bars have each a small tongue or rectangular fin forged on to the under side of the bar, and there is one corresponding channel cut in the transverse rod-bar. When the switches are properly closed in one position, the channel cut of transverse bar will be opposite one of the wire bar fins, and will allow one of the signals to be pulled over, but the other wire bar cannot be moved. The closing of the switches in the reverse position moves the channel cut so as to allow the other wire bar to be pulled through, but as there is only one channel cut in the transverse bar, only one signal can be pulled over for each position of the switches.
Throw-off or trap points, are introduced to throw an engine or train off the rails of a siding on to the ballast, and so avoid a collision with any other train which may be standing or passing on the line of rails with which such siding forms a connection.
[Fig. 484] is a diagram sketch of the arrangement, in which the main-line points are indicated by the letter A, and the trap points by the letter B; one series of rodding actuated by one lever in the signal-cabin works both the main-line points and the trap points at the same time and by the same movement. The connections are so made that when the points A are set for the passage of trains on the main line, the trap points B are set open to throw off on to the ballast, as shown in [Fig. 484]; and when the main-line points A are set to allow a train to pass from the siding on to the main line, the trap points B are closed, as shown on [Fig. 485]. A disc or other signal, worked or interlocked with the points, is placed near B to notify the engine-driver when he may pass out of the siding on to the main line; but should he from any cause proceed before the points are properly set and the corresponding signal given, his engine would run off at the ends of the rails C, C, and be derailed on to the ballast. The inconvenience caused by such derailment would be trifling compared with what might result from a collision with a train standing or passing on the main line. In some cases the siding is continued onwards for a considerable distance from the trap-point rails C, C, as indicated by the dotted lines D D, and terminates with a dead end. When this arrangement can be adopted, derailment is obviated, and the engine is brought to a stand by a buffer-stop at the end of siding. On no account should trap points be placed close to the top edge of a high embankment, or up to the abutment or wing walls of an under-line bridge, where an engine running through them accidentally might fall down a considerable height, and cause serious results. All sidings joining on to main lines should be trapped as above described, and when properly signalled and interlocked in the signal-cabin, the traffic-working can be carried on with increased facility and security.
[Fig. 486] is a sketch of an average sample of an ordinary single-arm wooden signal-post, with signal-arm, lamp, spectacles, ladder, and gearing complete for wire connection to signal-cabin. When the arm stands out in the horizontal position, representing the danger or stop signal, the red spectacles will be in front of the lamps, and will show a red light to an approaching train. When the arm is lowered, as indicated by the dotted lines, the second spectacle will be in front of the lamps, and will show either a white or green light (according to the accepted code) as