The origins of the Class 153 date back to 1990, when British Rail needed replacements for its ageing fleet of Class 121 and 122 “Bubble Car” diesel railcars for use on the quiet Cornish, Welsh, North-Western, Norfolk and Lincolnshire branch lines, as well as augmenting rural two car units. Originally built as a two-car Class 155 “Super Sprinter” by British Leyland between 1987 and 1988, using a construction technique similar to that used in the Pacers, pre-formed panels were riveted together, creating a lightweight body on a welded floor assembly.
Introduction was rapid, but teething troubles with the door mechanisms soon emerged, consequently the fleet was taken out of use and modified, with the Class 156 units taking over the duties.
The conversion to Class 153 was carried out by Hunslet-Barclay at Kilmarnock between 1991 and 1992, with 70 vehicles being produced. The layout of the original non-cab ends was different from the original cab end, so the ends are noticeably distinct with the vehicles not being symmetrical.
The large number of windows makes the vehicles look unusually long, being built on such a lightly built body has led to a slight sag being apparent in certain coaches over time.
As the conversion did not require any doors to be moved, the No.2 cab is significantly smaller than the original Leyland cab and encroaches on the door vestibule area, providing cramped conditions for Drivers and Guards. Similar to other Sprinter units, such as the Classes 150/2, 156 and 158, these units have gangway door connections at either end that allow passengers and staff to walk between vehicles working in multiple. These units also have the benefit of having passenger door control panels at either end of the cars.
72 passengers can be carried in 2+2 high backing, mixed bay and airline style seats, with 15.5m2 of floor space available for standing. Each vehicle is fitted with a toilet cubicle, as well as a large parcels locker, as at the time the units were constructed in the 1980s, a large quantity of local parcels traffic was still conveyed by rail under the Red Star Parcels brand. This fell out of use in the period leading to rail privatisation and the large areas are now used as luggage space.
Powered by a 285hp Cummins NT855-R5 Diesel, working through a Voith Hydraulic transmission, the maximum speed of a Class 153 is 75 mph, making it perfectly suited for small cross country services.
They are fitted with standard BSI auto-couplers, allowing them to work in tandem with other multiple units from the 14X and 15X Classes fitted with the same coupler, as well as newer units like the Class 170.
Units are numbered in the range 153301-335 and 153351-385. Individual cars are numbered 52301-335 and 57351-385 (originally 57301-335).