The S class locomotives were built with a 4-8-2 wheel arrangement, a configuration commonly used in Australia for heavy goods locomotives, smaller driving wheels giving increased tractive effort at the cost of reduced speed.

The vast majority of locomotives were well past their useful service life and many were badly in need of repairs.

The P and Pr classes had helped alleviate pressure on aging passenger locomotives when introduced in 1924 and 1938 respectively, but more powerful machines were needed with an order for 10 authorised.

The first three were built in 1943, with the remaining seven deferred while Midland Railway Workshops completed its order for 10 Australian Standard Garratts for the Commonwealth Land Transport Board.

After suffering from a range of early problems (which led to controversy involving the designer Frederick Mills), the class became highly popular and was very successful.