In 1916, Clyde Engineering were contracted to produce the K Class locomotives. The first K Class, 1353, entered service in 1918, but the NSWGR wanted extensive testing, so the next K Class wasn't produced for another 2 years. During this time, the order had been reduced from an original 300 locomotives to 120. All 120 locomotives were in service by 1925 and fitted with large capacity Wampu tenders. The K Class locomotives featured tapered boilers and the uncommon 'Southern Valve Gear'.

As part of the NSW reclassification scheme in 1924, the K Class became the D55 Class. At the beginning of their service the K Class / D55 hauled express fruit and mail, by the 1930s most of the class were converted to non-tapered superheated boilers and became known as 'Standard goods engines'.