speed. The upper terminal had sidings, with switching managed by ropes. London and Croydon The piston carriages were six-wheeled vans, with a driver's platform at each end, as they were double ended. The driver's position was within the carriage, not in the open. The centre axle was unsprung, and the piston assembly was directly connected to it. The driver had a vacuum gauge (a mercury manometer, connected by a metal tube to the head of the piston. Some vehicles were fitted with speedometers, an invention of Moses Ricardo. As well as a brake, the driver had a by-pass valve which admitted air to the partially exhausted traction tube ahead of the piston, reducing the tractive force exerted. This seems to have been used on the 1 in 50 descent from the flyover. The lever and valve arrangement are shown in a diagram in Samuda's Treatise. Variable size piston Part of Samuda's patent included the variable diameter piston, enabling the same piston carriage to negotiate route sections with different traction tube sizes. Clayton describes it: the change could be controlled by the driver while in motion; a lever operated a device rather like an umbrella at the rear of the piston head; it had hinged steel ribs. To accommodate the bracket for the piston, the traction tube slot, and therefore the top of the tube, had to be at the same level whatever the diameter of the tube, so that all of the additional space to be sealed was downwards and sideways; the "umbrella" arrangement was asymmetrical. In fact this was never used on the South Devon Railway as the 22 inch tubes there were never opened; and the change at Forest Hill only lasted four months before the end of the atmospheric system there. A variable diameter piston was also intended to be used on the Saint-Germain railway, where a 15 inch pipe was to be used from Nanterre to Le Pecq, and then a 25 inch pipe on the three and half per cent grade up to Saint-Germain. Only the 25 inch section was completed, so a simple piston was used. Engine house locations, South Devon Railway Exeter; south end of St Davids station, up side of the line Countess Wear; south of Turnpike bridge, at 197m 22c, down side Turf; south of Turf level crossing, down side Starcross; south of station, up side Dawlish; east of station, up