 In later years after they were allowed to cross the Flinders Street Viaduct between Spencer Street and Flinders Street stations, they worked goods trains of over 1,000 tons between Morwell and Melbourne, and even worked the South Gippsland line to Korumburra frequently until the late 1950s and as far as Foster.Even in its original form, the X class locomotive was a marked improvement on the C class in terms of performance.Comparative tests between prototype X27 and C class locomotive C18 revealed that the X developed an indicated horsepower output of between , compared with at for the C. With a later revised boiler design and other changes improving performance and increasing indicated horsepower to as much as , the X class was renowned for its ability to be driven extremely hard.