The Class 09 diesel shunters were a higher geared version of the very numerous Class 08. Just 26 were built and were capable of 27.5mph as opposed to the 15mph of the Class 08. As such they are useful engines for shunting and very occasional use in passenger or goods service.
D3721 entered service with the Southern Region of British Railways on 17th April 1959 where it is believed it spent its whole working life. It carried British Railways green livery when introduced, before painting in British Rail blue, during which time it was renumbered 09 010, then latterly the rather dull departmental grey livery, which it was still carrying when purchased for preservation.
D3721 was withdrawn and put into store at Hither Green depot on 5th February 2004, before being put up for sale on 19th April 2010 by DB Schenker (English Welsh & Scottish Railway) and purchased by the SDR and moved to Buckfastleigh in September of that year.