the expansion of melbourne's population into new suburbs early in the 20th century and the delay of the suburban electrification project saw the need for faster and more powerful steam locomotives for the suburban rail network. the basic design of the d was in 1908 adapted into a 4-6-2t tank locomotive, classed formula_1. they were put to work on longer and hillier suburban routes such as the dandenong, frankston, upper ferntree gully, williamstown, werribee, lilydale, darling and kew railway lines. a total of 58 were built between 1908 and 1913.