The front rod of the switch, that is, the rod which connects the points of the two moving rails of the switch, is pierced with two holes placed a distance apart just equal to the throw of the switch. In front of these holes is a bolt which is worked by a lever in the signal-tower. After the switch is set the lock-lever is reversed and the bolt enters one of the holes, thus securely locking the switch in position. There is one other interesting feature of this facing-point lock. It has happened very often that a switch has been thrown under a moving train, splitting the train and derailing more or less of it. This class of accidents is especially likely to happen when train movements are very frequent, and may be prevented by the use of the "detector-bar." This is a bar about forty feet long, placed alongside the rail, and carried on swinging links, like those of a parallel ruler, in such a way that any effort to move the bar lengthwise of the rail must raise it above the top of the rail. This bar is moved by the same lever which moves the locking-bolt. So long as there is a wheel on the rail above the detector-bar it cannot be moved, therefore the locking-bolt cannot be withdrawn, and the switch cannot be moved until the train has passed completely off it. [Illustration: Split Switches with Facing-point Locks and Detector-bars. (The rod on the right of the track is the mechanical connection to the lever in the signal-tower by which the locks and detector-bars are moved.)] [Illustration: Derailing Switch.]