The class D engines (of both the Drummond and Holmes types) were highly successful in operation, and had lengthy careers in service.
There are documented examples of even the NBR's own locomotive workshops failing to correctly distinguish between the J33 and J34 types of D class engine, owing to their near-identical appearance.
Drummond produced the class D on similar principles, but with a significant reduction in overall length and weight of the engine, thus permitting far wider route access for freight train operation.
The engines worked well, and although designed for light freight, were regularly employed on mainline passenger services, or wherever required on the network.
Owing to the close similarity, they were often identified colloquially as "Wee Drummonds", although the nickname properly belongs only to the J34 type.
It is suggested (for example, by NBR historian John Thomas) that the drivers found the Holmes cabs more pleasant to work in, and less draughty.
The later members of the class were actually fitted with the Westinghouse brake apparatus from new, an indication of the intention to employ them on passenger services.