Motor Rail was a British locomotive-building company, based in Bedford, England. Formed in 1911 as The Motor Rail & Tramcar Co Ltd, they built Petrol and from 1930 Diesel engined locomotives, mainly narrow gauge. During World War I over 900 locos were supplied for use on temporary military supply railways. The trade name Simplex was used from the early days; in 1931 the company was renamed Motor Rail Ltd, and in 1972 Simplex Mechanical Handling Ltd.
For further and more detailed information see Wikipedia at Motor Rail.
For those Simplexes that have figured on the FR and the WHR, see the category "Simplex" below.