The wheelbase was 6' 6" + 8' 3" + 10', some 18" longer than on the T9s. The boiler was based on that fitted to his double single N°720 of 1905 and the long firebox, too big to fit between the coupled axles, was pitched higher with a sloping grate to clear the trailing axle. Feed water heaters were provided and the boiler feed was by two duplex pumps on the frames between the coupled wheels. Contrary to his 4-6-0 designs, the D15s performed exceptionally well and were put to work on the Bournemouth run where, apparently, many drivers preferred them to the T14 4-6-0s as they used less coal, water and oil and were quicker and easier to prepare and dispose. Drummond did not have long to enjoy the class' success as he was involved in an accident in 1912 and died whilst the class was still being built. His successor, Robert Urie, built no more D15s as he preferred to concentrate on 4-6-0 designs. He was man enough, however, to admit in 1914 that the Drummond 4-4-0s on similar duties burnt far less coal than his H15 4-6-0s, though the H15 was, in turn, more economic than the Drummond T14 4-6-0s.

These successful locomotives worked, in the main, out of Waterloo and prior to electrification had a virtual monopoly of the Portsmouth expresses. Until the advent of the Bulleid light pacifics one was usually to be found working the Brighton to Plymouth service. N°468 differed from the rest of the class in that it had the safety valves mounted on the firebox, with Urie N15 style dome and safety valve casings, whilst the others had Drummond direct-loaded safety valves on their domes. As built, N°463 was fitted with a hooter rather than a whistle, which it kept until the second World War when it was removed to avoid confusion with the A.R.P. sirens.