I do differ with his comment that the 2 car units were for peak hour strengthening. My memory from the 60s is that 10 cars was the normal configuration, dividing at Thorpe-le-Soken with 6 cars to Clacton and 4 cars to Walton.
They are still my favourite EMU
Yes I agree, the stadard configuration (from the country end) was:
309/1 - 2-car
309/2 - 4-car griddle car set
309/3 - 4-car composite set
Whe they arrived at Thorpe-le-Soken, the leading 6-cars went straight to Clacton, leaving the remaining 4-car set to go to Walton-on-the-Naze, - a perfect arrangement that gave more time for the Clacton train to turn round and service the griddle car.
The reinforcement of trains in the peaks with additional 2-car units was as far as I remember limited to one peak down train in the evening, i.e. the 17:40 (1F78?). This train's consist was:
309/1 - 2-car
309/1 - 2-car
309/2 - 4-car griddle car set
309/3 - 4-car composite set
The train went to Clacton only I think. First stop was Colchester, it left London 10 minutes after the 17:30 to Yarmouth via Norwich, (class 47 + 11) and usually arrived at Colchester less than 2 minutes after it. Both trains were constrained by a fast between Liverpool St. and Shenfield and intermediate between Stratford and Harold Wood, both 75mph EMUs, on their way to to Southend Victoria, so the time was made up on the sprint after Shenfield.