That is the fundamental rule of the signal. Some roads have experimented with other forms of indicators--disks of one sort or another, semaphore blades that turn upwards rather than drop. The devices are numerous, but the principle is the same. When the tracks begin to multiply, and the signals begin to multiply in even greater proportion, they are generally carried over the tracks on a light bridge construction--our English cousins call it a "gantry"--and a series of small semaphore masts built up from the bridge. One of these masts, or "dolls," will be assigned to each track; and if there chances to be an unsignalled siding-track of little importance passing under the bridge, it will have its own "doll" rising from the bridge although quite devoid of semaphore blades. So it is all quite as clear as print to the engineer, even when forty or fifty lights blink at him from a single bridge. The signals tell their story to him quite as simply as to the man in the tower, who is setting their blades in accordance with his carefully arranged plans. Where signals are not of this interlocking type, guarding some junction, railroad grade crossing, draw-bridge or other point of possible danger, they are likely to resolve themselves into the block system. This system, in a rather crude form, with the use of operators at each block-tower or way-station, has been in development for something less than thirty years upon the American railroad. In brief, it divides a line--usually double-tracked, but sometimes used by the so-called "staff" method upon a single-track road--into sections, or blocks, of from three to five miles each. On double-track under this system, no two trains, even though travelling in the same direction are permitted in the same block. At the entrance to each block stands a tall mast with two of the conventional signal blades. The upper of these raised denotes that a train is still in the block, and an engineer must stop his train and wait till it drops, before he can proceed. The lower blade, when raised, indicates that a train is in the second block ahead, and the engineer must proceed only with caution and expecting to find that block closed against him. It is all quite simple; and if the engineers followed the signals absolutely, there never could be any rear-end collisions on lines protected by block signals. As a matter of fact, there rarely ever are, although the engineers do take chances time and time again. "Why should I stop for that thing," said a veteran engineer on a fast express train as we went whirring by one of those upper blades raised and commanding us in a blood-red point of light to stop, "when I can look down this straight stretch and see they're clear? Like as not something's got into the mechanism of it and let her flop that way."