|Number built:||26 cars (16 DMBS 10 trailers)|
|Engine:||2 AEC or Leyland 1595 diesels per DMBS|
|Power:||300 hp (220 kW)|
|Formation:||Driving Motor Brake Standard (DMBS)+[Driving Trailer Standard (DTS)]|
The Class 121s were allocated to British Rail's Western Region working on lightly used branch lines in Cornwall and along the Thames Valley. Originally in BR green they later carried BR blue and grey and some also received Network South East livery . In the privatisation era they have worked with Silverlink and Arriva Trains Wales as well as their current operator Chiltern Railways.
The Class 121 outlasted all other first generation DMUs and a small number are still in service having been fitted with central door locking to meet current safety standards . A number of Class 121s also found a new role in departmental service and were renumbered in the Class 960 sequence. They have been used for route learning, track surveying and sandite depositing. Three 121/960s remain in service, 5 DMBS and 2 trailers have been preserved to date.
|W55023 at Chinnor|
|Chiltern 960 014 at Aylesbury|
|W55024 / 960 010 at Chinnor|
 Brian Haresnape, Diesel Multiple Units, the First Generation (Ian Allan, 1985) p. 60
 Gavin Morrison, British Railway DMUs in Colour (Ian Allan, 2010) p. 33
 Colin J Marsden, DMU and EMU Recognition Guide (Ian Allan, 2013) p. 80