Mechanically similar to their predecessor the 44 Class, the 45 Class eschewed the streamlined exterior in favour of a hood design, which permitted more straightforward maintenance.

With a highly reliable and effective pedigree, the 45 Class were introduced as the pride of the NSW Fleet. Upon their debut in 1964, they took up service on leading passenger express trains of the day, including the venerable Southern Aurora.

While they operated right across the state, these powerful locomotives spent much of their working lives in the state's North and West, hauling both passenger and goods trains as far north as Brisbane, and West to Broken Hill.

Although excellent performers, the locomotives were not known for having ideal conditions for the crews inside. The hooded cab design ultimately made the cab uncomfortable, forcing hot air in during summers and cold draughts in winter. This, in addition to the bogie designs producing a rough ride, led to their eventual ban as leading locomotives.

To alleviate these issues, six of the class were modified and given cab upgrades. These updated locomotives were renumbered as 451 and then 35 Class locomotives - reusing the numbering previously held by their steam counterparts.