Britain at Work
A Pictorial Description of Our National Industries

258 block telegraph instrument indicates one or other of the following facts, viz.: “Line clear,” “ Train on line,” “ Une blocked.” Sometimes it is a needle which does this; sometimes it is a miniature semaphore signal; and sometimes it is an arrangement of differently coloured and worded shutters, which revolve round an aperture in a green blind. The normal position of the block instrument shows the line blocked. Now let us suppose that A wishes to pass a train into the section A B. He notifies B of his wish; and B, having ascertained that the section B c is clear signals back “Line clear.” As the train enters the section A B, A signals to B, “ Train on Jine.” B repeats that signal to C as the train passes his own box, and at the same time signals back to A, “Line blocked,” which A takes as a receipt, delivering him from all further responsibility for that particular train. The communications between any two signalmen are always heralded by bell calls, for which there exists one uniform code describing the nature of the train. Thus one beat calls attention; four, given consecutively, means“ Is line clear for express passenger?”; five, given 1 4, “ Is ]ine clear for express goods ? ” ; and two consecutively, “ Train BRITAIN AT WORK. entering section.” There are also arrange- ments of beats signifying “ Obstruction ’ Danger!” “Vehicles running away on wrong (or right) line,” “ Stop and examine train,” etc. With few exceptions, all bell signals must be acknowledged by exact repetition. But signalling alone cannot ensure the safety of a train. The track itself, bristling with switches, cross-overs, and sidings, must first be prepared for it. This is provided for by intricate mechanisms, which interlock the signals and switches, and thus establish the principle of the signalman being compelled to set a road before he can unlock it. Conse- quently the most wonderful mechanical part of a junction signal box lies hidden from sight underneath the lever floor, and here the maze of plungers, rods, and cranks have the effect of creating a system incapable of producing discord. Concerning the safety appliances that exist in the track itself for ensuring the correct working of switches, we have only space to. mention the facing point lock and duplex detector, which bolts the tongues of steel in one position or the other; the locking bar— a long flat bar lying along the edge of the rail, close to a switch—which, each time the switch is moved, must be raised above rail WATERLOO “ A ” SIGNAL BOX.