This rocking-bar consists of an angle iron or tee-iron bar of a length equal to the longest wheel-base of the rolling-stock, and is carried on short pivoted arms working in cast-iron or wrought-iron brackets secured to the rails as shown in Fig. 481. The pivoted arms have a movement backward or forward, and when at either the one or the other extremity, the upper surface of the rocking-bar is sufficiently below 318 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 By means of the rodding G, worked by a lever in the signal-cabin, the locking-bolt B is first withdrawn from the slot; the points are then pulled over into the reverse position by the rodding H, and the locking-bolt B is again set back into one of the slots by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.