Permanent Way (history) - Switches and Crossings

Switches and Crossings

Terminology is difficult for "switches and crossings" (S&C) previously "points and crossings", or "fittings".

Early S&C allowed only a very slow speed on the subsidiary route (the "turnout"), so geometrical design was not too important. Many older s&c units had a loose joint at the heel so that the switch rail could turn to close to the stock rail or open from it. When the switch rail was closed a reasonable alignment was secured; when it was open, no wheel could run on it so it did not matter.

As speeds rose this was no longer feasible and the switch rails were fixed at the heel end, and their flexibility enabled the toe end to open and close. Manufacture of the switch rails was a complex process, and that of the crossings even more so. Speeds on the subsidiary route were rarely higher than 20 mph except in very special designs, and great ingenuity was employed to give a good ride to vehicles passing through at speed on the main line. A difficulty was the common crossing where continuous support to wheels passing was difficult, and the point rail was planed down to protect it from direct impact in the facing direction, so that a designed irregularity in support was introduced.

As faster speeds were required, more configurations of s&c were designed, and a very large number of components, each specific to only one type of s&c, was required. At faster speeds on the turnout road, the divergence from the main route is much more gradual, and therefore a very considerable length of planning of the switch rail is required.

About 1971 this trend was reversed with the so-called vertical s&c, in which the rails were held vertical, rather than at the customary 1 in 20 inclination. With other simplifications, this considerably reduced the stockholding required for a wide range of s&c speeds, although the vertical rail imposes a loss of the steering effect and the ride through new vertical s&c is often irregular.