A diesel multiple unit or DMU is a multiple-unit train powered by on-board diesel engines. A DMU requires no separate locomotive, as the engines are incorporated into one or more of the carriages. Diesel-powered single-unit railcars are also generally classed as DMUs. Diesel-powered units may be further classified by their transmission type: diesel–mechanical DMMU, diesel–hydraulic DHMU, or diesel–electric DEMU.
Pacer was the operational name of the British Rail Classes 140, 141, 142, 143 and 144 diesel multiple unit railbuses, built between 1980 and 1987. They were inexpensively developed using a passenger body based on the Leyland National bus on top of a chassis based on the HSFV1 research vehicle. The railbuses were intended as a short-term solution to a shortage of rolling stock, with a lifespan of no more than 20 years. As modernised replacements were lacking, the Pacer fleet remained in service on some lines until 2021 – 37 years after their introduction in 1984.
The British Rail Class 108 diesel multiple units were built by BR Derby from 1958 to 1961, with a final production quantity of 333 vehicles.
The British Rail Class 121 is a single-car double-ended diesel multiple unit. 16 driving motor vehicles were built from 1960, numbered 55020–55035. These were supplemented by ten single-ended trailer vehicles, numbered 56280–56289. They have a top speed of 70 mph, with slam-doors, and vacuum brakes. The driving motor vehicles were nicknamed "Bubble Cars" by some enthusiasts.
The British Rail Class 116 diesel multiple units were built by BR Derby from 1957 to 1961. Introduced as part of the British Railways Modernisation Plan in the mid 1950s, as with other first generation DMUs the 116 was intended to replace steam trains and reduce costs across the rail network. Alongside Metro-Cammell, BR Derby had prior experience with DMUs, having developed a Lightweight Unit, and so was awarded a contract for a new design.
The British Rail Class 114 diesel multiple units were built by BR Derby from September 1956 to July 1957. Forty-nine 2-car units were built, numbered E50001-49 for driving motors and E56001-49 for driving trailers. The units were used in the early days out of 40A Lincoln TMD (LN) on services all over the county, although a small number were transferred to 41A Sheffield (Darnall) during 1959/60.
The British Rail Class 104 diesel multiple units were built by Birmingham Railway Carriage and Wagon Company from 1957 to 1959.
The British Rail Classes 101 and 102 diesel mechanical multiple units were built by Metro-Cammell at Washwood Heath in Birmingham, England from 1956 to 1959, following construction of a series of prototype units. These classes proved to be some of the most successful and longest-lived of BR's First Generation DMUs, second in longevity only to the Class 121, with the final five units being withdrawn on 24 December 2003. The oldest set was, by then, just over 47 years old.
The London, Midland and Scottish Railway (LMS) introduced a number of railcars to service between 1933 and 1939. Most were single units but one was a three-car articulated set.
The British Rail Class 144Pacer was a diesel multiple unit (DMU) passenger train built at Derby between 1986 and 1987. BR, seeking to procure improved derivatives of the earlier Class 141, placed an order with the manufacturers British Rail Engineering Limited (BREL) and Walter Alexander to construct their own variant, the Class 144. A total of 23 units were constructed. All units have now retired from mainline service, though the majority of the units have been acquired for preservation on heritage railways and in other uses. As of December 2022, 19 out of the 23 units have been purchased following withdrawal for this purpose, of which 15 units are in operational condition.
The British Rail Class 118 diesel multiple units were built by the Birmingham Railway Carriage and Wagon Company (BRCW) and introduced from 1960. It was a licence-built version of the British Rail Class 116.
The British Rail Class 100 diesel multiple units were built by Gloucester Railway Carriage and Wagon Company Limited from 1956 to 1958, designed and built in collaboration with the Transport Sales Dept. of T.I. Ltd.
The British Rail Class 155 is a diesel multiple unit passenger train. These DMUs were built by Leyland Bus at Workington between 1987 and 1988 as part of BR's replacement of its ageing first-generation diesel fleet. 42 units were originally built, of which only 7 remain; the other 35 units were converted to Class 153 railcars.
The British Rail Class 115 diesel multiple units were 41 high-density sets which operated the outer-suburban services from Marylebone usually to destinations such as High Wycombe, Aylesbury and Banbury which are on the Chiltern Main Line and Great Central Main Line. Sometimes, these sets used to operate 8- or 12-car-long expresses to Nottingham Victoria in the final years of the GCML. Coincidentally, Class 115 units operated services under Table 115 in the British Rail timetable.
The British Rail Class 140 was the prototype of the Pacer diesel multiple unit.
The British RailClass 141 was the first production model of the Pacer diesel multiple unit (DMU) railbus.
The British Rail Class 124 diesel multiple units were built by BR Swindon Works in 1960.
The British Rail Class 123 was a design of diesel multiple unit built for British Rail in 1963. They were the last first-generation DMUs built for British Railways and were built at Swindon Works, designed like all Swindon units as inter-city sets. Ten of the four-car sets were built and introduced in 1963. The units bear a visible similarity to the British Rail Class 309; however, there is no 'relation' here as the two types were built by different manufacturers for different markets.
The British Rail Class 125 was a design of three car Diesel Multiple Unit built by BR Derby at Derby Works in 1958. They were almost identical in appearance to the Class 116.
