British United Traction DMUs


The British United Traction produced various diesel multiple units.

Early British Railways DMUs

BUT produced 11 lightweight diesel multiple unit vehicles for British Railways during the 1950s, numbered 79740–79750 with an M at the beginning. the first units finished in 1952. When they finished being reviewed by British Railways, British Railways took them into stock in 1953. They were of three different types of rail cars, and could be made up into two- or three-car units. The first units were fitted with skirts covering their underframes and wheels; they were later removed. They were not very successful so they were all withdrawn by 1962, so never received TOPS classification.
Year BuiltDMS carsTS carsDMBS cars
1953797407974179742
19557974579746, 7974779743, 79744
1957797487974979750

First-generation DMUs

BUT also built the powertrain for the majority of British Rail 'first generation' DMUs.

Engines

The engines mainly had six horizontal cylinders and were made in several sizes:
AEC and Leyland engines were those designed and built by the respective companies and were not badge engineered except for BUT badges.
Six- and eight-cylinder Rolls Royce diesel engines were also used.
The first post-war lightweight DMUs 1954-1969 are reputed to have had pre-war Leyland 8.6-litre horizontal diesel engines fitted. It would be nice to have this confirmed.
Turbo-charged versions of the and engines were available, boosting the power outputs to and respectively. The turbo-charged versions were not used by British Railways, but they were used by Ulster Transport Authority.
Some engines were badged AEC, some Leyland and some Leyland-Albion.
Engines were genuine AEC or Leyland, each type carrying B.U.T. badges.

Transmission systems

Most units had Standard Mechanical Transmission. This comprised a fluid coupling, a freewheel and a four-speed epicyclic pre-selector gearbox. The freewheel facilitated gear-changing and also had an important safety function. If an engine seized, the freewheel would disengage and give protection against transmission damage and possible derailment.
A small number of units had hydraulic transmission, using a torque converter.

Coupling codes

The level of standardisation achieved with BR's first generation DMUs was much higher than it is with modern DMUs. All had side buffers, screw couplings and vacuum brakes and the majority had the standard Blue Square coupling code system for control of engine speed and gear-changing. There were some units with non-standard coupling codes but they were a small minority. The coupling codes were:
Some first generation DMUs were fitted with Rolls-Royce engines. Three types were used:
The six-cylinder engines usually drove through standard mechanical transmissions, but the eight-cylinder engines were usually coupled to Twin-disc torque converters.

Mixed running

Rolls-Royce devised a system which enabled DMUs with mechanical and hydraulic transmissions to work together. The British Rail Class 127 units were fitted with this system and given the standard Blue Square coupling code, instead of the Orange Star code given to the British Rail Class 125 units.
There was a gear-change lever on the driver's desk and it worked as follows:
It was a good idea but it created problems in practice. If the mechanical stock was at the back, the driver might forget to change gear, and this could lead to gearbox damage. In 1969 the Class 127s were re-labelled with the Red Triangle coupling code, and mixed running was discontinued.