The DEB railcar or 900/950 class trains are a Diesel Multiple Unit train built between 1951 and 1961 by New South Wales Government Railways and were used throughout NSW. The DEB sets were air-conditioned and intended for long distance main line services. The DEB sets were deployed in 3 and 4-car units consisting of a driving power car at each end and 1 or 2 trailer cars in the centre. Buffet services were provided from one of the trailer cars in each unit.


The first 4-car set, powered by four 250 horsepower horizontal in-line six cylinder, four stroke Hercules DFXH-F diesel engines (2 per power car) coupled to Torcon hydraulic transmissions, entered service in on the Sydney to Grafton North Coast Daylight Express service in November 1951. The engines and transmissions proved to be operationally unreliable and the set was withdrawn in May 1952 after less than 6 months in service. The construction of further vehicles was suspended until a satisfactory solution could be found. The updated power plant fitted was two 250 horsepower General Motors Detroit Diesel 6/110 inclined in-line six cylinder, two stroke diesel engines coupled to Allison hydraulic transmissions in each power car. Production work was resumed and two 4-car sets entered service in May 1955 on the Canberra-Monaro Express service (Sydney to Canberra and Cooma). This success of this service was followed up by the Far West Express (one 3/4-car set) in December 1957 and the Northern Tablelands Express (two 4-car sets and two 3-car sets) in June 1959. The Department of Railways' Water Supply Workshops at Chullora gained experience in lightweight bodywork whilst building aircraft during World War II. Consequently, these and all other in-house DMUs featured bodywork of largely aluminium construction. The underframe was of steel construction and steel Pratt trusses formed the body framing up to the window sill level. There were 92 vehicles built to this basic design between 1949 and 1968 including the 600 and 620 Classes. The Canberra-Monaro Express was reduced to a single 4-car set in July 1973 following a fall in patronage. The repeated failures of the 1200 Class Tulloch railcars working the Riverina Express resulted in the now spare Canberra DEB set being assigned for Riverina working in August 1973, allowing one Tulloch set to be withdrawn. The Far West Express service was withdrawn in September 1975 and assigned to Riverina working, allowing the second Tulloch set to be withdrawn. During the 1980's a program of retrofitting higher performance Cummins engines with Japanese Niigata lock-up torque converter transmissions to all units was commenced. Part way through the program, use of the Niigata transmission was dropped in favour of the Voith T 211 r model. Following the introduction of the XPT services from 1982, the DEB sets were gradually removed from main line services but continued to provide various XPT connecting services around NSW, including Junee to Griffith and Werris Creek to Moree. They were also used for outer interurban services on the Illawarra Line and Southern Highlands railway line. They were eventually retired when the Illawarra Line was electrified in 1993 and the CityRail Endeavour railcars were introduced from 1994. One DEB set is under restoration by Lachlan Valley Railway, based in Cowra


The DEB sets were able to provide very flexible main line services with a seven or eight car train operating from Sydney to a major junction station and then dividing into two trains to travel through to their destinations. On the return journey they were amalgamated at the junction and travelled through to Sydney as a single train. The newer CountryLink Xplorer railcars work similar services today on the Northern Tablelands Explorer, the train dividing at Werris Creek and travelling to Moree and Armidale. The power cars had two different configurations, the 900 Class (PF 901-910) having full passenger seating, while the 950 Class (HPF 951-958) had a compartment for 4 tons of luggage in addition to passenger seating. PF 906 was rebuilt as HPF 959 in 1982. The trailer cars had a number of different seating configurations, depending upon which of the services they were deployed. There were three First Class passenger cars (TB 801-803), seven Second Class sitting cars with buffer (TFR 851-857), two composite sitting cars with buffet (TCR 861-862) and three composite sitting cars (TC 751-753). There were also three parcel trailers built with dual braking systems to enable them to work with normal Westinghouse braked passenger stock and with the rail car fleet. These were coded TP 351-353 and worked through from Sydney on overnight mail trains to connect with the Far West Express at Dubbo and obviated the necessity of transhipping luggage and parcels between the mail and the rail car service. Power for lighting and air-conditioning was provided to the trailer from the adjoining power car.