Although not the very first diesel on the network, 4001 and the 19 classmates that followed wrought the legacy that mattered. A refined version of a classic Alco model, the 40 Class were assembled in Canada by an Alco affiliate, the Montral Locomotive Works, with 4001 and 4002 arriving in Sydney during the spring of 1951.

With traffic booming during the afterglow of World War II, the NSW Railways went on a locomotive shopping spree during the late 1940s and early 1950s, yielding the D58, D59 and AD60 Class steam locomotives for goods traffic. Mixed in there was the historic first order for mainline diesel locomotives: the 20-member 40 Class fleet.

Delivered in pairs over the ensuring six months, the 40 Class was to prove a true game changer. These locomotives could uncouple from a slow, heavy goods train and go straight onto an express passenger service - and excel at both jobs.

Gone was the need for locomotives designed for one task or another. Further, the new 'contraptions' did not require regular servicing, nor coaling and watering, and offered crews a level of comfort that, in the 1950s, was unimaginable when compared to the back-breaking slog of running a steam locomotive.

Built by A Goninan and Co in Broadmeadow, the six members of the 43 Class visually followed in the footsteps of the streamlined 42 Class, but with internals produced by the American Locomotive Company (ALCo) - a unique combination in any class of Australian diesel locomotive.

With its early diesel orders, the NSW Railways dallied with small orders from a handful of local and overseas builders, before all but settling on a fleet of near-standard Alco mainline diesels over the ensuring decades.

The 43 Class were introduced from 1956 with sights focused on their use on services between Sydney and Orange. With the electrification of the line to Lithgow in 1957 enabling electric-hauled services, the locomotives were transferred to work on the northern routes, taking over services at Gosford and travelling as far as the Queensland Border on both the North Coast and inland Main North routes.