Engineering Wonders of the World
Volume I

RAILWAY SIGNALLING. 233 pulls over the points SS ; while the right- hand slot operates valve D, and permits the air passing valve R5 to traverse pipes y and v to valve R3. High-pressure air is thus given access to cylinder I2, presses its piston up (stage 3), and so automatically completes the stroke of the slide. favour of mechanical signalling ; whereas in America high wages and long distances be- tween stations have favoured automatic sys- tems, which there protect more than 10,000 miles of track. The purpose of automatic signals is, of course, to replace the fixed signals operated Fig. 14.—DIAGRAM OF LOW PRESSURE PNEUMATIC We may notice that the completion of stroke cannot occur until the points have been shifted fully and locked. Also that the interlocking tappet rod, H, is raised or lowered by movements of the slide, L. During the reverse stroke of the slide the valves R2 and R4, and the pipes u and b (the last indicated by dotted lines), play their part. Automatic Signalling. Automatic signalling is the operation of fixed signals by the trains themselves, quite independently of any human agency. The principle has been adopted more widely in the United States than in Great Britain or on the Continent of Europe, for reasons which are too technical to be given fully here.* Suffice it to say that on this side cheap labour and a greater respect for safety have been in * See H. Raynar Wilson’s Power Railway Signalling for a detailed account. SWITCH AND LOCK MOVEMENT. by hand from a signal box, and thereby save the cost of a box, locking frame, etc., and a signalman’s wages. For operating the signal arms compressed air (high or low pressure), electricity, and carbonic acid gas are variously used; but whatever be the power, electricity is employed to switch it on. Part of the essential equipment for auto- matic signals is the track electric circuit. The rails are bonded to their neighbours by a light iron or copper wire, except at the ends of the sections, where the adjoining rails have to be isolated by insulated joints, so that current may not flow from one section to another. In Fig. 16 we show two complete sections, over which the train is supposed to be moving from right to left. At the ingoing (right) end of a section is the signal guarding that section ; at the outgoing end is the battery which