|Replaced||Steam locomotives and carriages|
|Number built||38 cars|
|Number preserved||3 cars|
|Number scrapped||35 cars|
|Successor||British Rail Class 121|
British Rail Class 122
|Operator(s)||Great Western Railway|
|Line(s) served||Western Region|
|Maximum speed||63 mph (101 km/h) to|
80 mph (130 km/h)
|Track gauge||4 ft 8+1⁄2 in (1,435 mm) standard gauge|
In 1933, the Great Western Railway introduced the first of what was to become a very successful series of diesel railcars, which survived in regular use into the 1960s, when they were replaced with the new British Rail "first generation" type diesel multiple units.
The original design featured streamlined bodywork, which was very much the fashion at the time. The rounded lines of the first examples built led to their nickname: "flying banana". The preserved W4W is an example of the original, rounded body shape. Later "razor edge" examples, such as No. 27 (pictured), had much more angular (and practical) bodywork, yet the nickname persisted for these too.
The interiors of railcars No. 1 to No. 18 were heated by using waste heat from the engine cooling water. This system proved unreliable in service due to issues with the thermostat valves employed. Later vehicles from No. 19 onward abandoned this system to its unreliability and because their revised specification demanded that they be able to haul steam heated rolling stock. 
These later vehicles, No. 19 onwards, were equipped with steam heating systems. Which were capable of heating both the vehicles interior and that of any additional trailed vehicles, via a steam supply to the standard steam heating equipment. Steam was produced by a fully automatic Vapor Clarkson steam generator manufactured by Gresham and Craven Ltd under licence from the Vapor car co. The generator was of the water tube with counterflow arrangement type, and could supply up to 300 lb of steam per hour, at 45 lb per square inch. The quantity of steam supplied could be varied by the driver to suit the demand required by a solo railcar, or plus one, or plus two, trailer vehicles. The fuel used was the same diesel used for the railcar's engines. However, it was stored in a separate 45 gallon tank. The water supply for the heating system consisted of a single 100 gallon tank.
Railcars No. 1 to No. 18 were powered by a high speed diesel engine manufactured by A.E.C, producing a maximum brake power output of 130 hp (97 kW). The engine was of the straight 6 configuration, with a bore of 115 mm diameter and a stroke of 142 mm. This gave a total displacement of 8.85 litres. The maximum operating speed was 1,800 rpm.
Railcars No. 19 onwards were powered by a modified version of the previous engine. This engine was equipped with direct injection and the bore diameter was enlarged to 120 mm. The stroke remaining at 142 mm. This engine produced a lower brake power output of 105 hp at 1,650 rpm.
An unusual feature was the external cardan shaft drive from the gearbox on the rear of a horizontally mounted engine to road-vehicle style reduction boxes outboard of the two axles on one bogie. Later units had two such engine and drive combinations placed on opposite sides. Railcars 19-20 were fitted with a separate high-low ratio gearbox on the final drive side of the gearbox. This allowed a top speed of about 60–70 mph (97–113 km/h) in high and about 40–45 mph (64–72 km/h) in low. Railcar W20W retains this in preservation.
The brake system on railcars No. 2, 3 and 4 was unconventional. Instead of the usual vacuum actuated tread brakes used on most british rolling stock of the period. An automotive style system was adopted, utilising vacuum -hydraulically actuated drum brakes. A vacuum brake cylinder - hydraulic master cylinder set was mounted on each bogie. From the master cylinder, hydraulic fluid passed through hoses to the operating cylinders. The operating cylinders actuated cam mechanisms within the brake drums to apply the internally expanding brake shoes to the inside circumference of the brake drums. The 20 inch internal diameter cast steel brake drums were bolted to the inside face of one wheel per axle. The vacuum necessary for brake operation was created by 3 rotary exhausters. Two being driven directly from the engines, one by each engine. The third exhauster was chain driven by the final drive shaft. This arrangement allowed vacuum to be maintained during coasting, when the engines would be shut down. The system operated at a vacuum of 22 to 24 inches of mercury. This vacuum being stored in four reservoirs on the chassis. Another unusual aspect to this system was that vacuum was increased in the vacuum brake cylinder to apply the brakes. This being contrary to normal british railway practice, in which vacuum is reduced to apply the brakes.
The prototype unit, No. 1, made its first run on 1 December 1933 between London Paddington and Reading with a large number of press representatives. Three days later this unit entered public service between Slough, Windsor and Didcot.
Soon after this the GWR ordered the next three production units, nos. 2 to 4, which were built with two engines (instead of one) which allowed them to reach a maximum speed of 80 mph (130 km/h), and included a buffet. These units were delivered in July 1934 and entered service on 15 July 1934 between Birmingham Snow Hill and Cardiff General. This was the first long distance diesel express service in Britain, and covered the 117.5 miles (189.1 km) miles between Birmingham and Cardiff in 2 hours 20 minutes. This was intended as a businessman's service, fares were charged at the normal rate, however bookings were limited by the 44 seats of the railcar.
The next three units, nos. 5 to 7, entered service in July 1935 and had 70 seats. These were used on services between London, Oxford and Hereford. The next batches of railcars numbered 8 to 34 were of various different designs and entered service in batches between 1936 and 1941, two of these (nos. 17 and 34) were designed for express parcels services rather than passenger services.
The earlier units operated as single railcars. The final four, numbered 35 to 38, were twin coupled units with the driving cabs situated at the outer ends of the set, these were in effect the forerunners of today's diesel multiple units (DMUs). These had the capacity for 104 passengers, however a standard corridor coach could be installed between the two cars, and this could increase the seating capacity to 184. These were introduced in November 1941 and worked the Birmingham-Cardiff service.
|Number Range||Introduced||Builder||Engine||Weight (long tons)||Seats||Withdrawn||Notes|
|1||1934||Park Royal||1 AEC of 130 hp (97 kW)||24 long tons (24.4 t; 26.9 short tons)||69||1955||Prototype railcar|
|2–4||1934||2 AEC of 130 hp (97 kW)||26.2 long tons (26.6 t; 29.3 short tons)||44||1954–1958||Buffet fitted|
|5–7||1935||Gloucester RCW||25.3 long tons (25.7 t; 28.3 short tons)||70||1957–59||Standard single car|
|8–9, 13–16||1936||29.5 long tons (30.0 t; 33.0 short tons)||70||1957–60||9 withdrawn in 1946 after fire|
|10–12||1936||29.9 long tons (30.4 t; 33.5 short tons)||63||1956–57||Lavatory fitted|
|17||1936||28.85 long tons (29.31 t; 32.31 short tons)||None||1959||Parcels car, capacity 10 long tons (10.2 t; 11.2 short tons)|
|18||1937||33.6 long tons (34.1 t; 37.6 short tons)||49||1957||Prototype, with buffers & draw gear for hauling vans|
|19–33||1940–41||GWR, Swindon||2 AEC of 105 hp (78 kW)||35.65 long tons (36.22 t; 39.93 short tons)||48||1960–62||33 rebuilt in 1954 to replace 37|
|34||1941||34.9 long tons (35.5 t; 39.1 short tons)||None||1960||Parcels car, capacity 10 long tons (10.2 t; 11.2 short tons)|
|35–38||1941–42||36.7 long tons (37.3 t; 41.1 short tons)
+ 37.6 long tons (38.2 t; 42.1 short tons)
|60 + 44||1957, 1962||Power twins with buffet and lavatory 35+36 and 37+38 |
37 withdrawn in 1949 after fire and replaced by 33
Five of the 38 railcars were destroyed by fires:
- No 9 was burnt out at Heyford in July 1945; officially condemned in May 1946
- No 10 was burnt out at Bridgnorth in March 1956; officially condemned in May 1956
- No 35 and No 36 were destroyed by fire at St Anne's Park, Bristol in April 1956; officially condemned in April 1957
- No 37 was damaged by fire in February 1949 and was stored until being scrapped; officially condemned in September 1949
Three of the GWR railcars have survived into preservation, which are as follows:
|Vehicle no.||Builder||Year built||Location||Comments||Photograph|
|W4W||Park Royal||1934||National Railway Museum, York||Static Display|
|W20W||GWR Swindon||1940||Kent & East Sussex Railway||Under restoration at Tenterden since 1983.|
|W22W||GWR Swindon||1940||Didcot Railway Centre||Operational.|
Hornby Railways manufacture a model of the 1940-style railcar in OO gauge, using tooling acquired in their takeover of Lima. In late 2017, Dapol released an OO model of the streamlined 1936 Gloucester RCW railcars in a variety of liveries and numbers. Graham Farish has produced an N-gauge model (with various numbers, e.g. 19, 22, and 20), both before and after their takeover by Bachmann.
- Judge, C. W. (2008). The history of Great Western A.E.C. diesel railcars. Southampton: Noodle. ISBN 978-1-906419-11-0. OCLC 233788958.
- Handbook for railway steam locomotive enginemen. Shepperton: Ian Allan. 1998. ISBN 0-7110-0628-8. OCLC 59990926.
- "Great Western railcars. The Park Royal bodied railcars, numbers 1 to 4". The Great Western Archive. Retrieved 2 March 2020.
- "Great Western railcars. The Gloucester bodied railcars, numbers 5 to 18". The Great Western Archive. Retrieved 2 March 2020.
- "Great Western railcars. The Swindon bodied railcars, numbers 19 to 38". The Great Western Archive. Retrieved 2 March 2020.
- Judge, Colin (2008). The History of the Great Western A.E.C. Diesel Railcars. Kevin Robertson (Noodle Books). p. 227. ISBN 9781906419110.
- "Back home! Swindon legends go back on display". The Railway Magazine. Vol. 161, no. 1, 377. Horncastle, Lincs: Mortons Media Group. 2 December 2015. p. 9. ISSN 0033-8923.
- "Hornby GWR Diesel Railcar". Hornby Railways Collector Guide. Retrieved 29 March 2020.
- Allen, Geoffrey Freeman (February 1983). "The GWR's Flying Bananas". Rail Enthusiast. EMAP National Publications. pp. 26–31. ISSN 0262-561X. OCLC 49957965.
- Riley, Dick (May 1983). "Railcars remembered". Rail Enthusiast. EMAP National Publications. pp. 32–35. ISSN 0262-561X. OCLC 49957965.