to be used on the Original Proposed East Coast Main Line Electrification Scheme, however, when this was abandoned the units were deployed on Great Eastern Main Line (GEML) express services from London Liverpool Street to Clacton-on-Sea and Walton-on-the-Naze (also marketed as the Sunshine Coast Line) which is what gained them the class nickname Clacton Express. Trains would be formed of three units in a ten-car formation (i.e. one two car unit, and two four car units). The train would divide at Thorpe-le-Soken, with one of the 4-cars units used on the Walton section, and the remaining six cars continuing to Clacton. The Clacton portion usually included the griddle car unit, and was always the second to leave Thorpe-le-Soken, being the rear (London end) portion of the coastbound train. The choice of building units with only two coaches may seem unusual, but these were intended only for use in strengthening pairs of four coach units at peak times to produce a ten coach train, and with this in mind they only had second class accommodation. 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. In the 1970s many of the units were reformed. The two-car units were augmented to four-car units. In 1973-74 units 605-608 with the addition of TSK with TCK converted former Mk1 SK and CK coaches which formed in to 4 car sets. In 1978 units 601-604 with the addition of TCK and TSOL converted former Mk1 CK and TSO coaches which formed in to 4 car sets. In the early 1980s units 611-618 had the griddle cars were taken out of use and replaced by TSOL from 601-604 and TSK from 605-608. 601-608 were augmented to 3-car units until refurbishment work in 1985-86 In 1986-87 refurbishment work required the addition of 8 Mk1 TSO coaches to be converted to TSO trailers for units 611-618 with the borrowed TSOL and TSK trailers all being converted to TSOL trailers and reformed back into 601-608. The