Engineering Wonders of the World
Volume I

RAILWAY SIGNALLING. 229 impossible, nor can signal 2 be pulled “ off ” for a train to pass along the main line when the points 4 4 are open for a shunting move- ment. The operation of levers in their proper sequence is also secured. By this is meant that lever 7 for the branch-line signal cannot be pulled until after 8 for the branch-line points, nor signal 5 for leaving the siding until the siding-points 4 4 are opened, nor the distant signal 1 until the stop signals 2 and 3, to which it applies, are “ off.” By the same means the distant lever must be put back to danger before the stop signals, lest the distant signal might indicate “ clear ” when the home signal is at danger; and the siding signal must be put to normal danger—before the siding-points are closed. There are two systems of interlocking— lever-locking and catch-handle locking. Lever-locking is explained by Figs. 8 and 9. Fig. 8 is a side view of a lever and a . . . . tappet rod, C. Lever-locking. The second of these passes through a box, X, its movements being re- strained by locks. The mean- \ ing of these locks is shown in -Quaiant Fig. 8. Fig. 9, where we look from above, as it were, on four levers with their tappets, C1, C2, C3, C4. Running crosswise above the tappets are Fig. 9. slide bars, to which the locks A, B, and D are attached. If tappet 2 were moved down the page, the sloping top of notch N1 would push lock A to the right into notch. N2 of tappet 3, which could not therefore be moved until tappet 2 had been restored to its original position. Similarly a movement of tappet 3 would lock tappet 2. If levers 2 and 3 operate stop signals on two converging lines at a junction, it is evident that the signals cannot both, be put to “ clear ” simultaneously. Turning now to tappets 1 and 4, we see that these are influenced by the locks B and D, attached to a common slide bar. In this case tappet 1 cannot be moved from its normal position until tappet 4 has been moved to bring notch N4 into line with D. If tappet 1 be then pulled over, tappet 4 will be held from returning to its normal position. This sequence of operations is known as back- locking, as it prevents one lever being put