The introduction of the Deltics was a step change in locomotive performance on the East Coast Main Line. The recently introduced Class 40 diesels had an absolute maximum drawbar horsepower of 1,450 and this could be exceeded by a Pacific steam locomotive if worked hard. On one of O. S. Nock's first Deltic runs (the down "Heart of Midlothian" loaded to 13 coaches, 530 tons gross) he states "once the tail of the train was over the 60 mph restriction the throttle was opened to the full, and the surge forward could be felt in the cab. Never previously had I felt a positive thrust in my back when in the second man's seat!" Nock went on to estimate that at 80 mph the locomotive was producing about 2,750 drawbar horsepower.