The class was designed for mainline passenger and freight use with more power than the D51 and a higher maximum speed than the C62 steam locomotive classes. This was achieved by installing two 1,100 hp engines in an 18 metre long centre-cab design, unusual for mainline operation. The V12 DML61 engines were developed from the 6-cylinder inline DMF31 engines used in the DD13 locomotives.
Locos numbered from DD51 501 to 799 and from 1001 to 1186 were equipped to operate in multiple, and locos numbered DD51 800 to 899 and 1801 to 1805 were built without steam generators for train heating.
All locomotives numbered from DD51 2 onwards were finished in the standard diesel livery of orange/red with grey upper surfaces separated by a white stripe. Re-engined locos operated by JR Freight in HokkaidÅ sport a livery based on the DF200 colour scheme, with no white stripe. These locomotives are frequently used in pairs double-heading freight trains. JR HokkaidÅ locos are all finished in the “Hokutosei” livery of blue with a gold stripe and shooting star logo. These are used in pairs for hauling sleeper trains (Hokutosei, Cassiopeia, and Twilight Express) between Hakodate and Sapporo.
DD51 592 (now withdrawn), and now DD51 791, was repainted in the “Euroliner” livery of pale blue with dark blue stripes for use with JR Central’s “Euroliner” Joyful Train set.
DD51 842 is designated as the Imperial Train locomotive. Whereas regular members of the class have white handrails and edges to the running boards, they are polished stainless steel on this particular locomotive, as are the exhaust shrouds. Based at Takasaki Depot, it is also used for special excursion trains.
As of April 2003, there were 138 DD51s still in service, with the vast majority operated by JR Freight. Of these, many have recently undergone life extension refurbishment, which includes removal of steam generator equipment where still fitted. These locos are distinguished by a new livery of blue with grey upper surfaces separated by a white stripe, and cream end panels.
DD51 class locomotives also formed the basis for the DD17, DD18, and DD19 self-propelled snow plough units.