Engineering Wonders of the World
Volume I

RAILWAY SIGNALLING. 237 post in the rear. Thus, D3 can- not go to clear till S3 has been cleared. Furthermore, a distant cannot be pulled off until the stop on the same post has been cleared, so as to avoid conflicting signals. The operation of the signals is explained by the six stages of Fig. 21. Stage, 1.—Train about to enter section 2. All signals clear. Stage 2.—Enters overlap X. Signals S2, D3 to danger. Sectton 4 lOverlipZi ' I Section 3 lOverlapY | Section 2 i Section 1 (Overlap X J I 1---------------I .. -X--------------1-- - —X---------- - -1------X-----±2------ s* ' S3 D4' s2l>3 ' 2 --------------I-------«----------—I--------, , I---O K ------------ S4 0s' S3 D4 ' S2 03 * 3----------------.--- --X-------------T0-*-----------------—X- ------------------ S4D5' <J3 S2Ds 4----------------1------x-----------0_(-------x-------------------.K- ~ - - ■--- S*D* s3D4 ' SJb3 ' 5--------------------------------1----1-------x----------‘--I------x---—-------- S4 D5 S3D4 SaD5 6----------------------------Q-l-X----1------*---------. --1-------X-------- - - S*DJ S3D4 S* D3 ' Fig. 21 . Fig. 22.—AN ELECTRIC TRAIN STAFF HOLDER. SS, STAFFS IN SLOT OF THE INSTRUMENT. Leaning against the side of the cabin is a staff showing the key K at the end for unlocking siding-points on the section. The engine-driver cannot remove the staff until the points have been locked again. Stage 3.—Enters section 3. Signals S3, D4 to danger ; hold S2 and D3 at danger. Stage 4.—Passes out of overlap Y. S3, D4 still at danger, but S2 is cleared. D3 remains at danger, being held by S3. Stage 5.—Enters overlap Z. S4, D5 to danger. Rear signals as before. Stage 6.—Passes out of overlap Z. S3 is cleared and releases D3. (Train in position of maximum danger.) Control of Single Lines. In the early days of railways, trains on single lines were worked to a programme which assigned certain places for trains to pass one another. If a train got behind time serious delays occurred, and the traffic was dislocated. An improved system therefore was introduced, whereby the line was divided into sections, to each of which was allotted a train staff bearing on it the names of the stations at both ends of the section. This had to be handed to the driver of a train as a sign of permission to enter a section, and surrendered by him at the other end. On lines where several trains might follow one another through a section, the staff was supplemented by tickets handed to the drivers of all but the last train, which carried the staff. The ticket-box being controlled by a