It is particularly important to notice that after the signalman at the advance end of a block has “plunged” his plunger remains locked and it cannot be released until the train admitted to the block covered by the plunger has completely passed out of that block, permitting the track-treadle at the entrance to the next block to unlock the plunger. This feature makes it impossible for one train to enter a block until the preceding train has passed out of it. If the permissive system of using a block be employed, in which the train is permitted to enter that block before a preceding train leaves it, the treadle gives no protection against a rear-end collision with the first train. In such an exigency other devices must be used or the following train must proceed cautiously, expecting to find the track occupied. =280. Distant Signals.=—Thus far the distant signals have been treated incidentally only. They may be operated concurrently with or independently of the home signal in such a way that if danger is indicated, the distant signal gives its indication prior to that of the home signal. In this manner protection is given to the rear of a train approaching a block against the home signal set at “danger.” After the obstruction is removed and the block cleared, the home signal is set at “safety” before the distant signal is cleared. =281. Function of Advance Signals.=—The advance signals are used when for any purpose it is desired to form a short block in a regular block. If, for instance, block 3 in Fig. 11 were obstructed by a train stopped by some failure of a locomotive detail, a train approaching station _B_ in section 2 against the home signal _Sʹ_ set at “danger” would be obliged to stop before entering block 3. It might then be permitted to enter the latter block, to be stopped by the advance signal _Aʹ_ set at “danger” or under instructions to pass it cautiously, expecting to find the track obstructed. It is thus seen that the advance signal creates what may be called an emergency block, and in reality finally controls the movement of trains in the block in which it is located. It would never be cleared unless the home signal were first cleared, nor would it be set at “danger” unless the home signal gave the same indication. The preceding operation of the block system of signalling controls the movement of trains along a double-track line. [Illustration: FIG. 14.]