- Sleeper (1)
- Sleepers have two functions: holding the rails at the correct distance apart (Gauge
 ) and transferring the pressures of passing trains from the rails to the ballast. Usually made of pressure treated hard wood, but softwood is also used and more recently pre-stressed concrete. LNWR experimented with pressed steel sleepers but did not adopt them. Often sleeper density is increased on curved track, where the lateral forces are higher.
- Sleeper (2)
- A colloquial term for a sleeping car (or sleeping carriage).
- Sleeping Car
- A passenger carriage with facilities for the passengers to sleep, on payment of a supplementary fare, originally only for first class passengers; after the grouping third class sleeping cars were introduced. LNWR and WCJS sleeping cars, and the LMS carriages of similar design, ran on six-wheel bogies, and were so solidly built that it was almost impossible to hear, or feel, the rail-beats when running.
- Sleeping Composite
- A hybrid WCJS vehicle 50ft in length, half ordinary carriage and half sleeping car, provided on trains where demand did not justify a full sleeping car. Curiously, no attempt was made to merge the styles: the sleeping portion looked like part of a sleeping car and the carriage portion like an ordinary passenger carriage.
- Sleeping Saloon
- The legend on LNWR and WCJS sleeping cars was “SLEEPING SALOON”; strangely, this appeared not on the waist-band, as in dining cars, but above the windows.
- Slide Bar
- A straight piece of metal fixed to the back of the cylinder block but parallel to the bore of the cylinder, on which the Cross Head slides. Often two or four were used for each cylinder. Maybe of any shape, but rectangular is most common.
- Slide Valve
- In its general form, a slide valve resembles an upside-down rectangular pie-dish in the steam chest and moved to and fro by thevalve gear. Pressure of steam in the steam chest holds the valve with its narrow rim against a smooth flat face pierced with three slots (called ports). The outer slots are connected by passages to the ends of the cylinders, the central slot communicates with the blast-pipe. Movement thus uncovers no more than two slots at a time,allowing expanded steam to be exhausted from each end of thecylinders in turn. Such valves were almost universal in steam engines until super heating demanded something better and the piston valve was adopted.
- Slip Coach
- A railway carriage which can be uncoupled (slipped) by its guard from the back of a moving train and coast under his control to stop at the next station.
- Slip Point
- A combination of a diamond crossing and either a pair of points, allowing movement from one of the crossing lines to the other (single slip), or two pairs of points, allowing movement from either of the crossing lines to the other (double slip) – also known as a single or double compound.
- Slotted Signal Post
- In some early signals the signal arm was pivoted in a slot in the post rather than on the side of the post facing an oncoming train. Originally, the arm was concealed in the slot at “off” (clear), but some slotted posts remained in use after the practice of the arm being lowered to about 45 degrees became general. Slotted post signals had drawbacks: the arm could stick in the post due to ice and show a false ‘off’; arms could blow off or be damaged in severe weather and this also would show a false ‘off’. Such signals were phased out and replaced by normal semaphore signals which showed a positive 'clear' indication.
- Slotter
- See Frame Slotter.
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