The Class 309 units were considerably more powerful than the Britannia pacifics that they replaced with a typical 10-coach formation (2 x 4-car units plus 1 x 2-car unit) producing 3,384 hp for tare. However, from 1971 it was found that additional capacity was required for the evening peak and the 17.40 departure from Liverpool Street station was increased to 12 coaches by the addition of a second 2-car unit bringing the power output up to 4,512 hp from a train of tare.