Renowned for their power and speed, in the ten years that followed their introduction the running time of the premier Sydney express service they operated was progressively reduced by one and a half hours.

They were also equipped with long-range tenders to enable the entire 190+1⁄2-mile (306.6 km) journey to be run non-stop at a speed that remained for the next 20 years Australia's fastest train service.

They ran up annual mileages double that of other classes of locomotives on the VR and by 1954 had run a combined total of approximately 5,700,000 miles (9,200,000 km).

Although Annual Reports show the railways planned to upgrade the main Western line and build further S class locomotives to haul The Overland services between Ararat and Serviceton, exigencies imposed by the Great Depression and World War II meant the planned upgrade of that service never eventuated.

After ten weeks of trials, it was regularly rostered on the key North East line Sydney Limited and Albury Express services unassisted, hauling trains comprising up to ten E type carriages plus luggage van over the 1 in 50 gradients that carried the line from 30 to 1,145 feet (9.1 to 349.0 m) above sea level in its first 33+1⁄4 miles (53.5 km).

On 29 April 1928, S300 went on public display at Flinders Street station in conjunction with fund-raising efforts for the building of the Shrine of Remembrance.

The superior power of the S class had a profound impact on the timetable which, in July of that year, saw a cut of half an hour from the previous five-hour nineteen-minute northbound Sydney Limited schedule of A2-hauled trains.

With the line speed limit raised to 70 miles per hour (112.7 km/h), the S class-hauled Sydney Limited was reported as being the fastest train in the Southern Hemisphere,[30] with the southbound service running at an average speed of 48 miles per hour (77.25 km/h), including the five-minute stop at Seymour to take on water.

[32] When first constructed, the trailing delta truck axle under S300 had insufficient lateral damping and, when running at high speed around curves, tended to knock the track out of the ideal transition spiral alignment.

[33] In April 1935, S303 was equipped with the VR's Modified Front End, to improve drafting and reduce cylinder back pressure.

That caused a problem, which was exacerbated by the reduced ventilation following the fitting of the streamlining associated with the Spirit of Progress service from 1937 onwards, coupled with the faster running speeds involved.

[38] From November 1937, the S class was assigned to haul the VR's luxurious, all-steel, fully air-conditioned streamliner, the Spirit of Progress.

At the time, they were the only passenger locomotives on the VR with enough power to take the Spirit's eleven-car trailing load of 544 long tons (553 t) unassisted over the 1 in 50 gradients between Melbourne and Albury.

[39] Preparations for the S class to haul the Spirit of Progress were made as early as February 1937, when S301 was fitted with streamlined cladding that dramatically altered its appearance, together with a long-range tender, with capacity for the 13,000 imperial gallons (59,000 L; 16,000 US gal) of water and 8.5 long tons (8.6 t) of coal which enabled the train to travel the entire 190.5-mile (306.6 km) journey non-stop, at an average speed of 50 miles per hour (80 km/h) northbound and 52 miles per hour (84 km/h) southbound.

[41] While initially painted wagon red, by November both streamlined locomotives were repainted in the royal blue and gold livery, designed to match the new Spirit of Progress carriages.

[43] In the context of the British locomotive Mallard reaching a recorded 126 miles per hour (202.8 km/h) in the following year, the speed of the Spirit was unremarkable, but the publicity surrounding the launch of the new streamliner, including footage of the train racing against an aircraft, captured the public imagination nevertheless, and was widely reported in contemporary press and newsreels.

[44][45][46] Locomotives S300 and S303 continued to haul Melbourne to Albury services in their unstreamlined form until converted during major overhauls in March and October 1938 respectively.

[48] A minor change to the original streamlining was the removal of the solid cowcatcher and its replacement with a lattice type, after it was discovered that the original design created a partial vacuum behind the cowcatcher, which sucked dust and grime into the slide bars for the centre cylinder, creating maintenance problems.

S 301 was temporarily repaired with a semi-streamlined appearance, featuring a standard VR pressed-steel slotted cowcatcher and a non-streamlined smokebox.