The Layout of the Motor Rail Simplex Locomotive

 This diagram below shows the layout of a plate-frame 20/28 H.P. loco, but the general layout is similar on all the company’s “Simplex” designs and originated with the 20 H.P. Petrol Tractors built for the War Department Light Railways. This loco is a left hand drive, i.e. the seat is arranged so that the gear levers are operated by the driver’s left hand. In some models, the seat was positioned on the opposite end of the loco, thus becoming right hand drive. The reasons for different models having either left or right hand drive are not known with any certainty, although it is likely that in the earlier models the arrangement was dictated by the position of the speed control lever on the engine.

 The transversely mounted engine and gearbox can be seen along with the chain final drive to the wheels. The standard wheels were made of chilled cast iron, a material that is very hard wearing but, when it finally does wear, is virtually impossible to machine, as many owners of preserved locos have found to their cost.

Below: The bare frame of a welded frame 20/28 H.P. loco seen from below. The slots in the central plate on the left hand side are for the linkages from the brake column. It is on the top of this plate that the works number can be found stamped on locos built after c1936. When ex-War Department locos came for reconstruction and their works plate was missing and their number unknown, a new number would be allocated and this can be found be stamped on the left hand side of the rear engine bearer. Works plates on most locos can be found on top of the rear (left on this picture) engine bearing cross-member. Some petrol locos, 20/26 H.P. for example had the works plate on the plate between the front sand boxes due to the engine timing retard/advance lever being mounted in the place usually taken by the works plate.

Click on the labelled parts for more information then click "return" to come back to the diagram.

 Detachable Tube Radiator

All but the very early radiators on Simplex locos were of the Davies Patent type and were arranged so that tubes could be replaced or repaired individually. This is something that is quite an advantage over other types such as Rustons, where the whole radiator had to be replaced and a fairly major strip down was needed to get at the radiator. (return)

 Change Direction Lever

As its name implies, this operated the forward/reverse feature of the gearbox. (return)

 Change Speed Lever

This lever selected the gear ratio. Normally only two speeds were available, but on the larger models such as the 32/42 H.P. and 60S there was a third gear. There was no synchromesh on the gearbox, as on motorcars of the time, and “double de-clutching” was required when changing down a gear. (return)

Clutch Pedal

Early locos employed an inverted cone clutch, an unfortunate characteristic of which was a tendency to stick when worn. This was replaced in later models by a flat clutch plate, with the 48/63 H.P. locos having a double clutch plate. The foot-operated clutch pedal was a disadvantage on systems where many turnouts had to be negotiated as the driver was constantly climbing on and off the loco. The loco could not be driven whilst walking alongside. (return)


Four of these were fitted, one for each wheel and they were operated by foot pedals. (return)


These operated on all four wheels by use of a brake wheel and screw. Cast iron brake blocks were provided. Originally a handwheel was provided for applying the brakes. In some later locos a lever was provided, this being much easier and quicker to apply in an emergency. (return)


These employed gun-metal bearings and were lubricated by sprung oil pads. Leaf suspension springs were used. (return)

 Standard Universal Coupling & Spring Buffer

This was first introduced in 1920 with loco 2014. It was a solid casting and usually had three slots for different coupling heights but some had five such as the straight-framed 20 H.P. petrol locos. A spring buffer was fitted to clear the coupling and this provided some shock absorption between the loco and its load. (return)

 Skid bar

This was provided to stop the wheels from sinking too far into the ground in the event of a derailment. It was made from tubular steel but was not all that strong and many extant locos show the signs of derailment - a bent skid bar. (return)

Optional Fittings.

As optional extras steel tyred wheels, whistles, cooling water circulating pumps and electric lighting and starting could be provided. Cabs could also be fitted, which came in several variations, the most obvious being the roof - curved or pointed. Cabs could also be retrofitted to locos purchased without a cab; these being obvious by the engine cover design. Engine covers designed for cabbed locos are supplied with the cab side missing. Tropical-type cabs and full length canopies could also be fitted. (return)

The Dixon-Abbott gearbox initially provided two speeds in each direction, with later versions providing three speeds for the heavier locomotives. Each gearbox comprised three parallel shafts, the Simplex layout negating the need for right-angle bevel gears. The input shaft was driven direct from the engine, and the output shaft carried the duplex drive sprocket for the driving chains to the wheels. Each shaft ran in roller bearings with the speed selection gears being in constant mesh. The different speeds were engaged using sliding dogs on the output shaft, whereas the direction selection was effected using a sliding primary gear on the input shaft. (return)

Exploded diagram of a three speed Dixon-Abbott gearbox as fitted to 60S loco.