We left the signalman trying to lower signal No. 2; vainly, because No. 3 lever was still normal and the switch unlocked (Diagram, p. 205). Probably he would not have begun his operations in the bungling way that has been supposed, but would have first reversed lever 3. That locks the switch by the facing-point lock, and locks also switch-lever 4 in the frame in the signal-tower and releases lever 2. Then he reverses lever 2. That locks lever 3 and releases lever 1. Then he reverses lever 1, which locks lever 2. Now the way is made for a train to pass east on the main line, and the signals are clear. The last signal could not have been lowered until the chain of operations was complete; none of the levers can now be moved until lever 1 is again put normal and signal 1 made to show danger. There is one point of great danger in this particular train-movement which has not been mentioned; that is, the crossing of main-line east-bound track _B_ by the branch-line west-bound track _C_. It will be noticed that with the levers normal, derailing switch 5 is open, and it is impossible for a locomotive to pass beyond it. Lever 5 is interlocked in the tower with lever 4 in such a way that, before 5 can be reversed to let a train pass west from _C_, lever 4 must be reversed to trap any train on _B_ and turn it down the branch _D_. It must not be understood that the use of "derailers" is universal. In fact, they are not recommended by the best signal engineers, except in special conditions. In the absence of derailer No. 5, signals 11 and 12 would be interlocked with switch 4, so that, so long as that switch stands open for the main line a clear signal cannot be given to a train coming west on _C_. It will be noticed that signal 2 carries two semaphores on one post. The upper one is for the main line and the lower one for the branch. Both are operated by one lever, 2, and whether reversing lever 2 lowers the main-line signal or the branch signal depends on the position of the switch. The switch is made to pick out its signal by an ingenious but very simple little arrangement, called a selector, which is placed somewhere in the line of ground connections.
It would be an interesting study, were there space, to follow the possible and proper combinations of movements to pass trains over the various tracks. It will be seen that, by concentrating the levers which move switches and signals in one place and interlocking them, it is made mechanically impossible for a signalman to give a signal which would lead to a collision or a derailment within the region under his control. The only danger at such points is that an engineer may overrun the signals. This description of the objects and the capacity of the system of interlocking is no fancy sketch. The system has been in use for many years, doing just what has been here described, and more. A recent close estimate gave the number of interlocked levers now in use in the United States as about eight thousand, and the number is rapidly increasing. Recent official reports showed that in Great Britain and Ireland there were thirty-eight thousand cases in which a passenger line was connected with or crossed by another line, siding, or cross-over. In eighty-nine per cent. of these cases the levers operating the switches and protecting signals were interlocked.