Engineering Wonders of the World
Volume I

RAILWAY SIGNALLING. 235 Stage 2 3 Section 3. J Overlap Ci Section2. ; Overlap B i Sectionl. Overlap A ' Brake TriggerSet ^Signals —< 2* fSignal 1 .* L ^Signal 2. ■ o\ -x 1 ’ —% 1 X -r - --x r 1 Minimum a O Protection 4 T ' Trigger douin --------1 T Q I* Maximum Protection Fig. 17. No. 1 is apparently in immi- nent peril. But this is not really the case. Opposite each signal by the side of the rail, is a stop or trig- ger (Fig. 20, A), which rises into a vertical position when the signal is at danger, and lies Automatic Brake Application. whereby, though the signal guarding a section is put to “ danger ” immediately the train enters that section, the signal at the entrance of the section just quitted is not put to clear ” until the train has proceeded a certain distance into the new section. This distance, known as the “overlap,” is 400 feet on the Metropolitan and Tube Railways of London. The simple diagrams of Fig. 17 will serve to explain the use of the “ overlaps ” A, B, and C, which form part of sections 1, 2, and 3 respectively. The portions of the track which are guarded by signals behind are in every case marked in full lines, and the “ cleared ” portions by dotted lines. Stage 1.—Train has passed overlap A. Signal 1 at danger. Stage 2.—Train enters second section. Sig- nals 1 and 2 both at danger. Stage 3.—Train passes out of overlap B. Signal 1 cleared ; signal 2 at danger. At this point the train is most exposed—that is to say, it might be only just over 400 feet from the signal protecting it. Stage 4—Train at farther end of overlap C. Signals 2 and 3 both at danger. The train is now in a position of maximum safety. To revert for a moment to stage 3. Let us sup- pose that one train has stopped just beyond the overlap, and that the driver of a train behind has failed to observe that signal 2 is against him, and overruns it. Then train Fig. 18.—AUTOMATIC SIGNAL ON NORTH-EASTERN RAILWAY.