The D55 class was a class of steam locomotives operated by the New South Wales Government Railways in Australia. They were built with the 2-8-0 wheel arrangement.
A contract was given in 1916 to Clyde Engineering Company of Granville for the construction of 300 K-class locomotives. Following experiments with Southern type valve gear on an earlier class, Edward Lucy, the Chief Mechanical Engineer of the N.S.W.G.R., proposed its installation on these locomotives. The use of Southern valve gear was rare on locomotives operating outside the United States of America. The first of the class, then classified as K1353, entered traffic on 29 November, 1918. Due to financial difficulties at Clyde Engineering, the next member did not appear for a further two years. Meanwhile, the contract had been reduced to just 120 locomotives. All were in service by March, 1925 and were fitted with a Wampu type tender. The last thirty of the class were fitted with self-cleaning smokeboxes and had other modifications.
The members of this class spent most of their days attached to depots at Enfield, Goulburn, Harden, Junee and Cowra. They were seldom used on western or northern lines. With the discontent and industrial action in the coalfields following World War II, it was decided in 1946 to convert some of the class to oil burners. The 55-class was chosen as the outside valve gear gave more room for the installation of the new equipment, which included altered firebox and smokebox. The tenders were fitted with a 2,400-imperial-gallon (11,000 l; 2,900 US gal) fuel tank. The fuel oil was injected into the firebox by a jet of steam from the locomotive boiler, the flow being controlled by the fireman. The first six locomotives converted were fitted to burn distillate which was five times the cost of coal firing, although it was hoped that reduced servicing times would offset some of that extra cost.
When cheaper crude oil became available, the locomotives were again modified to allow them to burn this heavier product. This required the installation of heating coils in the tank and pre-heating adjacent to the burner to ensure complete atomisation. A further 10 locomotives were converted in 1947, followed by another 54 in 1949. As the crisis passed, the oil burning locomotives were withdrawn as they were still four times more expensive to run than the coal fired ones. 16 were converted back to coal firing. The last oil burning 55-class was 5591 which was withdrawn in February, 1959. A distinctive feature of the oil burning locomotives was a hinged lid provided over the chimney to protect the boiler tubes and flues from sudden cooling when the oil fire was cut off. The last country depots to have 55-class locomotives were Goulburn and Cowra. 5597 was the last of the class in service, being attached to Enfield depot until June, 1967. Of the Standard Goods engines, the 55-class were thus the first to be retired. The Southern valve gear did not give as good steam distribution as was desirable, making the locomotives a little sluggish when hauling heavy loads, compared to the similar classes.
5595 is the sole survivor. It has been cosmetically restored and is displayed with a Commonwealth Turret Tender at the New South Wales Rail Transport Museum.