Introduced in 1925, the 75 locomotives of the class became the principal motive power for all major expresses, and accelerated long-distance passenger timetables leading to new levels of service in the pre World War II period.

The design was a response to the increased loading of express trains and public demand for shorter journey times, as well as to the need for reduced servicing and lower maintenance costs.

Typical of the technology of the day, the riveted steel boilers originally fitted to the class had copper inner fireboxes, fire-tubes and superheater flues.

Although copper provides superior heat transfer, it promotes a galvanic reaction resulting in the 'wasting' by corrosion of some steel boiler components such as crown stays and front tubeplates.

A noteworthy feature of the class was their outside Walschaerts valve gear, by then well-established in Britain and Australia and valued for its ease of lubrication and maintenance.

The increased range of the class was trialled early on, with through trips from Sydney to Albury, a distance of some 400 miles, without changing the locomotive.

Withdrawal of the 36 class commenced in January 1959 as more diesels became available, but a number were temporarily returned to traffic in late 1966, to work wheat trains after a record harvest.