The first 8/12 appeared in 1936. At only 2-Tons, this was one of the smallest Motor Rail designs. The design had a number of unusual features, although it adhered to the usual Simplex layout. It was one of only two Simplex designs to be fitted with a single cylinder engine, this being the Ailsa Craig CF1 diesel with Acro-type combustion chamber. This is the single cylinder version of the engine fitted to 16/24 H.P. locos. The engine was cooled by a water tank rather than a radiator, this being carried at the front of the loco giving it a very distinctive appearance, immediately distinguishable from other types. Another unusual feature not seen on other types was the running gear arrangement, which had the axleboxes running in adjustable horn guides, the boxes themselves supporting the frames on volute type suspension springs rather than the leaf springs usually found on Motor Rails. Most other features were similar to larger locos, including a two speed reversing gearbox, single dry plate clutch and braking applied by handwheel to all four wheels. Only twenty-three of the type were built, eight of these for the River Great Ouse Catchment board. Only three were exported.