Assuming a train approaching from A to proceed in the direction towards B, then, before the signalman in the NORTH cabin can lower the UP home-signal C, the signalman in the SOUTH cabin must first pull over his lever and release the slot which retains the signal C at _danger_, and in doing so the levers in his own cabin will stand locked, and prevent the lowering of the signal D, or opening of points E to allow access from the sidings to UP main line. The cross-over road F G will also be locked for main line clear. When the slot has been released from signal C, the signalman in NORTH cabin can lower the UP home signal C, but before he can pull over the lever for this purpose he must first lock the points H, to prevent access from the sidings to the UP main line, and also the points K L of the cross-over road, to keep the main line clear. A similar operation has to be gone through for a train approaching from B to proceed in the direction towards A, when the signalman in NORTH cabin must first withdraw the slot from the DOWN home-signal M before the signalman in the SOUTH cabin can lower that signal. A small automatic disc is placed in the cabin to indicate to the signalman when the slot has been withdrawn by his colleague in the neighbouring cabin, and for facility of working, the two cabins are usually placed in communication with each other by telegraph or telephone. At some stations similar to the above, where there is a very frequent train service, with several of the trains running through without stopping, it is the practice to have a second or lower arm to the home signals C and M, as shown on the diagram, these lower arms being only _pulled off_ for through or non-stopping trains, as an indication to the engine-driver that the line is clear in the section ahead. In addition to the leading signals shown in the sketches, there 328 are shunting signals for the movement and marshalling of trains--setting-back signals in connection with the making up of passenger trains; taking on or off passenger carriages; or moving out empty passenger carriages; and many other special signals which become necessary for the working of a large and complicated train service.