Numerous experiments were carried out with a view to determining a rail-wheel diameter calculated to give the most satisfactory results. It was assumed that the average Ford car attained a maximum degree of efficiency when running at a speed of 25 m.p.h. with the engine turning over at 1500 r.p.m. With a diameter of 2 feet 6 inches at the tread of the tyres, the road wheels and back axle would be revolving at the rate of 280 r.p.m. A further calculation went to show that with sprockets having a gear ratio of 30 to 40, and with the rail-wheels having a diameter of 18 inches, the latter would revolve at the rate of 210 r.p.m. as against the 280 r.p.m. of the Ford back axle, this resulting in the tractor averaging a speed of 987 feet per minute, or 11 to 12 m.p.h. which was considered adequate and suitable for the varying conditions to which it was likely to be subjected. In addition to ensuring a systematic means of transport for men proceeding to and from the trenches, the Crewe tractor was further requisitioned for taking supplies of ammunition to artillery emplacements in the forward areas. Suitable trolleys were attached, and the little tractor, prior to going into commission, was required to prove itself capable of hauling a dead-weight minimum load of 5 tons, not only on the level, but on an upward gradient of 1 in 20, halfway up which gradient it was further required to stop and re-start, there being in Crewe Works a track specially laid for the purpose of subjecting every tractor to this crucial test. It was found, by experiment, that by doubling the diameter of the trolley wheels, from 7 inches to 14 inches in diameter, double the load could be hauled. "Slipping," an inherent difficulty due to greasy rails, had to be reckoned with, and was in no small measure counteracted by the addition of a central driving chain, coupling rear and front axles through the medium of sprockets. The frictional resistance (_i.e._ the force at the rails when the wheels are on the point of slipping) was found to be 448 lbs. with a load of 1 ton on the carriage of the tractor, this being sufficient to enable the tractors to pass the required test. The Ford transverse rear spring was supplemented by two helical springs placed vertically between the Ford and tractor frames. To obviate the necessity of turning at the various termini of the track, and to enable the tractor to always proceed in forward gear, an ingenious method was devised, whereby, with the aid of a screw and ratchet, working on a transverse beam and socket laid across the rails, the tractor was raised bodily clear of the rails, and swung round, ready to proceed in the opposite direction.