Although it has a central cab that is most often seen on shunters, the DD51 is a diesel locomotive for mainline passenger and freight traffic. Enough power was ensured by two 1,000-hp and later 1,100-hp engines, one on each side of the cab. Special was the wheel arrangement with three two-axle bogies of which only the outer two were powered by hydraulic transmissions. The central bogie was needed to keep the axle load low with 15 tonnes. Of 649 built, all but the first 53 were equipped for multiple working and all but 110 had a steam generator for passenger service.

Although they only had a top speed of 95 km/h, they were also used for express services. Over time their use in passenger service decreased, so that JR Freight was the largest operator of the DD51 with 137 when the rail system was privatized in 1987. There most lost their steam generators in a refurbishment. Between 2004 and 2012, six were sold to Myanmar. Two more were sold to Thailand. In 2016, only 29 were remaining in service in Japan. Seven were preserved, including the prototype.