The facility and precision secured by the interlocking machinery enabled other valuable accessories to be introduced for the more complete signalling and protection of train-working. Amongst these may be mentioned the facing-point bolt-lock and rocking-bar, signal-detectors at points, and throw-off or trap points. With the old-fashioned hand-worked switches the man standing alongside could see whether the sliding-rails were properly closed, and also when the last vehicle of the train had passed over them; but when important main-line-facing switches or points are worked by rodding from a signal-cabin some distance away, it is necessary to have some reliable means to ensure that the sliding-rails are actually brought close home, and also to prevent the switches being moved again until 316 the entire train has passed over them. A set of switches may be carefully made and work well, but it is quite possible for some fracture or obstruction, to intervene and prevent them closing properly. If a train or engine were passing through them in a trailing direction, as indicated in Fig. 345, the wheels would most probably force the sliding-rail home, and no disturbance would arise. If, however, the train were coming in the opposite or facing direction, the chances are that some of the wheels would take one road and some the other, and cause a derailment. The same casualty would occur if the switches were moved during the passage of the train. To guard against the above contingencies, the facing-point bolt-lock and rocking-bar have been introduced. The system is applied in various forms, but the arrangement shown in Fig. 480 will explain the principle generally. A strong casting, A, is securely bolted to the top of the sleeper carrying the chairs on which rest the point ends of the sliding-rails. This casting has an internal groove or chamber formed for its entire length from C to D, as indicated by the dotted lines, and in which slides the locking-bolt B. The point ends of the switch or sliding-rails are connected by the transverse rod E, which is forged into a vertical bar form for that portion of its length, which passes through the opening, F, prepared for it in the casting A. In this vertical bar a hole or slot is cut to correspond to the exact size of the locking-bolt B, and at a distance to suit the sliding-rails when pulled over to their properly closed position. This locking-bolt, B, will not pass through the hole in the vertical bar until the sliding-rails are quite close home, and when once through the hole the sliding-rails cannot be moved until the locking-bar is withdrawn. In some cases two holes or slots are cut in the vertical bar to enable the points to be bolt-locked for both directions. The rocking-bar is designed to prevent the withdrawal of the locking-bolt before all the vehicles have passed over the points. [Illustration: Fig. 480, 483, 481, 482]