The single car DMU was built in 1960 and follows the success of the Gloucester RC&W single car units built in 1958 to replace the ageing GWR railcars. Later to become Class 121, their design included high density seating with doors to each bay to augment the Pressed Steel Class 117 three car units in the peak hours and to operate singly on lightly used branch lines on the Western Region of British Railways.
The units were originally fitted with B.U.T. (AEC) engines of 150bhp which were later replaced with Leyland 680 type engines. There was a total of 16 units built numbered from W55020 - W55035. Our W55023 is the oldest out of the five Class 121 single units so far preserved. It spent a considerable part of its life working on the lines out of Paddington allocated to Reading depot, and in March 1993 was repainted back into 1960s BR Green livery and relocated to Bletchley depot for use on the Bletchley to Bedford branch line operated by North London Railways. The unit arrived at Bletchley from Doncaster works on 5 April 1993.
In 1996 new operational policies meant that class 117 power twin sets were preferred to running single cars in pairs on the Bletchley to Bedford route and W55023 was declared surplus to requirements after its last turn of duty on 9 November 1996.
W55023 arrived at Chinnor on 27 March 1997 and is owned by the Association and is in working order. She made her first passenger run in preservation on 18 May 1997 and was repainted in November 2003 by Cranmore Traincare & Maintenance Services Ltd. W55023 is particularly used on the Railway's History and Nature rides and also for training purposes, charters etc.