By 1923, the A2 class 4-6-0 locomotives, which dated back to 1907, were frequently double-heading on interstate expresses to Serviceton and Albury as increasing traffic saw loads exceed the eight car maximum of a single A2 on these services. The Victorian Railways Commissioners recommended considerably more powerful locomotives to haul trains of up to eleven cars unassisted over the ruling gradients on these lines. The S class 4-6-2 Pacifics displaced the A2s from North East line express services from 1928 onwards and allowed a faster timetable to be introduced. However a Pacific-type locomotive was not particularly well suited to the Western line. The section between Melbourne and Ballarat had sharply curved, steep inclines, the most notorious of which was the 10-mile (16 km), 1 in 48 (2.08&) ruling gradient of the Ingliston Bank. As early as 1923 VR locomotive designers were considering 4-8-2 Mountain-type locomotives for this purpose, along with the use of a third cylinder to allow increased power to be achieved without breaching the loading gauge.
In 1936, the Victorian Railways Design Office finalised the major design requirements for a steam locomotive that was capable of hauling a load of 550 long tons (560 t; 620 short tons) at a minimum 20 miles per hour (32 km/h) up Ingliston Bank. In order to develop the power required, a very large grate and a very large boiler were needed, and the 4-8-4 wheel arrangement enabled this by using a four-wheel trailing truck to support a large firebox, and four coupled axles to support a large boiler and ensure a good factor of adhesion.
VR designers wished to improve on aspects of the earlier S class design. These three-cylinder Pacifics, although capable of hauling heavy loads at high speed, had proved to be relatively maintenance-intensive, particularly with regard to the servicing of the valve gear and motion for their third (inside) cylinder. The Gresley conjugated valve gear, driven from the valve spindles of the outside cylinder piston valves, was prone to heat expansion and wear, causing timing difficulties, and required dismantling and removal whenever the centre cylinder valve required service.
To this end, the H class had a significantly different arrangement for its third cylinder. A German Henschel & Son conjugated valve gear mechanism, driven from the combination levers of the Walschaert valve gear for the outside cylinders, was utilised in favour of the Gresley mechanism. The inside cylinder was also positioned further forward of the outside cylinders and drove the leading coupled axle, with the outside cylinders driving the second coupled axle.
The H class also became the first VR locomotive to feature a mechanical stoker, and boasted many other modern features such as roller bearings, hydrostatically controlled load compensating brake gear on the tender, power-operated reversing gear, American-style bar frame construction, thermic siphons, and duplex blast pipes.
Construction of three H class locomotives at Newport Workshops commenced in 1939 and three sets of frames were manufactured. However work was halted due to the outbreak of World War II. Due to a shortage of motive power caused by increased wartime traffic, completion of class leader H220 was authorised and the locomotive entered service on 7 February 1941. Streamlining similar to the S class was originally planned, but wartime economies saw this abandoned.
The two additional, partly built H class locomotives remained incomplete while wartime production of armaments (and later postwar rebuilding of badly run-down infrastructure) took precedence over express passenger locomotive construction. They were never completed, and the parts were subsequently scrapped.
Although it had been built to work the Western line to Ararat, a number of bridges along the route required strengthening before the H class locomotive with its 23¼ ton axle load could enter regular service on that line. The necessary work was deferred due to wartime restrictions on available resources. Therefore, H220 was put to work on the only line able to accommodate its loading gauge and high axle load, the North-Eastern line to Albury, where it hauled express passenger services, fast goods services, troop trains, and on the odd occasion when the regular S class Pacific was not available, the Spirit of Progress, particularly in the postwar period when S class locomotives were affected by poor coal and reduced availability. H220 gave an indication of its capabilities on one such run of the Spirit in the late 1940s by reportedly topping the 5-mile (8 km) long 1 in 50 Glenroy Bank at 45 mph (72 km/h) (three times the typical S class-hauled speed at this point,) going on to pass Seymour 13 minutes ahead of schedule and arriving at Albury at 20 minutes ahead of the 10:40 pm scheduled arrival time.
H220 never operated in its intended role as power for The Overland, although it did make a brief appearance on the Western line in 1949 when it ran a series of trials with the VR dynamometer car on goods trains from Melbourne to Ballarat. Results from the dynamometer car showed that the locomotive developed around 3,300 drawbar horsepower (2,460 kW) at 47.5 mph (76 km/h), and a starting drawbar tractive effort of 52,000 lbf (230 kN). The Australian Railway Historical Society, in listing the introduction of H220 among its '100 defining aspects of Australian railways' noted test results as high as 3,600 horsepower (2,680 kW) at 50 mph (80 km/h) were recorded, a power output unequalled in Australia during the steam era and only equalled in the modern era by the National Rail NR class.
Although intended for express passenger use, H220 was found to be particularly effective when used as a fast goods locomotive. It typically hauled five return trips a week between Melbourne and Wodonga, covering around 1,875 miles (3,017 km) per week and hauling loads of up to 820 tons (833 t) over the 1 in 50 gradients of this line. By comparison, the maximum loads of the C class 2-8-0 and X class 2-8-2 heavy goods locomotives between Wodonga and Melbourne were 555 and 650 tons respectively. H220 was also able to complete the journey in a much shorter time, with its large tender requiring only a single stop for water rather than the two stops for water and coal the other locomotives required. Even after the introduction of mainline diesel-electric traction from 1952 onwards, Victorian Railways promotional literature featured H220 prominently. One advertisement in Walkabout in November 1953, headed 'Trains we are proud of', promoted H220 as "Australia's mightiest engine" and its nightly service hauling the 21:25 Albury Interstate Fast Freight.
From all accounts, H220 was a success even though it spent its life hauling services and operating on a line it was not specifically designed for. Although one-off locomotives are often consigned to a short operating life or underutilised due to non-standardisation of parts and maintenance requirements, H220 remained in service until finally being superseded by diesel electric locomotives in the late 1950s. In a service life of 15 years 3 months, it clocked up a total of 821,860 miles (1,322,660 km), averaging over 4,800 miles per month.
Even after the war, upgrades on the line to Ararat necessary for H class operation were not made as the VR struggled with a backlog of maintenance work which had built up during the 1940s. When the VR finally embarked on major capital investment in the early 1950s, diesel and electric traction were under consideration. Although diesel locomotives of the early 1950s typically had far lower power outputs than the H class, they could be run in multiple-unit operation, with one crew controlling two or more locomotives. In August 1952, two new B class diesel-electric locomotives established their credentials during trials by hauling a 1,121 ton goods train up Ingliston Bank and covering in 44 minutes the same distance two A2 class locomotives took to haul a 690-ton load in 67 minutes. The B class locomotives proved their ability to provide the same (if not better) performance as the H class, but without the heavy axle load and requirements for upgraded infrastructure. They became the new motive power not only for The Overland, but also mainline goods service. The days of mainline steam locomotives were numbered.
H220 continued in service until it was withdrawn for an overhaul on 20 May 1956. By this time, the C and X class heavy goods locomotives were being progressively withdrawn from service and scrapped as B class diesel-electric and L class electric locomotives made them redundant. H220 was stored rather than overhauled, and never returned to service. It was written off the VR locomotive register on 30 April 1958.
H220 survived more-or-less intact until 1960, when the Australian Railway Historical Society successfully lobbied for the establishment of a railway museum. H220 entered the Australian Railway Historical Society Museum at Williamstown North in 1962, and since this date has been its star exhibit.
In April 2008, 50 years after its official withdrawal from service, H220 was added to the Victorian Heritage Register. Heritage Council chairman Chris Gallagher noted that H220 represented the peak of steam motive power technology in Victoria and warranted the state's highest level of heritage protection.
It is believed that H220 is now the world's only remaining example of a three-cylinder 4-8-4.
A number of manufacturers have produced brass models of the H class:
Alco produced multiple runs of the model in the 1980s (as it was then preserved in the museum);
Trainbuilder recently released two variants, for 1941-1950 and 1950-1958.