Motor Rail was a British locomotive-building company, originally based in Lewes, Sussex, they moved in 1916 to Bedford. Loco manufacture ceased in 1987, and the business line sold to Alan Keef Ltd of Ross-on-Wye, who continue to provide spares and have built several locomotives to Motor Rail designs.

The origins of the Motor Rail company can be traced back to the patenting of a gearbox by John Dixon Abbott of Eastbourne in 1909 ("Change speed and reversing gearbox suitable for use in motor-trams", UK Patent 18314). In March 1911, he formed The Motor Rail & Tramcar Co Ltd, with his father John Abbott and brother Tom Dixon Abbott. The stated aim of the business was developing the gearbox and incorporating it in tramcars and railcars. At about the same time operations moved to Lewes, Sussex and rented space in the Phoenix Foundry of John Every, where they developed a narrow-gauge rail vehicle around the Dixon-Abbott gearbox using a twin cylinder water-cooled Dorman engine.

In January 1916, the company answered a War Department tender for military supply railways. The specification was for a 600 mm (1 ft 11+5⁄8 in) gauge locomotive, with no more than 1 ton of axle load per axle, capable of hauling up to 15 tons at 5 miles per hour (8.0 km/h). The company designed a new locomotive, with outer longitudinal "bent-rail" frame, mounted on two driven axles. The 2JO petrol engine manufactured by W.H. Dorman & Co of Stafford was centre-set in the frame along with its Dixon-Abbott patent gearbox, which drove the unsprung axles through a chain-drive. At one end of the frame the operator sat facing to one side allowing him to drive equally well in either direction, and at the other end was the silencer and the water cooling radiator mounted with fan to provide transverse air flow. A large flywheel gave relatively smooth operation.

After approval by the War Department of the prototype, a small order was placed for evaluation in France. Success here led to significant orders and as a result the company moved to new premises in Houghton Rd, Bedford in May 1916. John Abbott died in August 1916, and John Dixon Abbott took over as chairman.

A second but armoured version was made to a specification defined by the Ministry of Munitions. This used an upgraded Dorman 4JO 40 hp engine and two speed gearbox, coming in three versions:

The Dorman engine used by Motor Rail from the earliest days was reported in 1918 to have been "designed by Mr Abbott, in conjunction with W.H. Dorman Ltd of Stafford", which could explain why the company exclusively used this engine. The same article states that the WW1 production of the Dixon-Abbott gearbox was contracted out to David Brown Ltd. of Huddersfield.

With over 900 such locomotives supplied to the WD and MoM, post-WW1 the resultant large and cheap supply of these trench tractors opened up the use of internal combustion engine powered locomotives to many new and existing applications, where steam engines were either too heavy or too expensive thus allowing cheaper operations. The company had found its niche, and continued to build petrol and diesel-engined locomotives, mainly for narrow gauge railways.

In 1931, the company changed its name from Motor Rail & Tram Car Co to Motor Rail Ltd. The trade name Simplex was registered in 1953, though the name Simplex was in use by the company as early as 1915. In 1972 the company was renamed Simplex Mechanical Handling Ltd.