It looks like a lengthy train, the Loco' is displaying the normal commuter train disc headcode for the period and a Bricklayers Arms Duty Number upon the white disc above the right hand buffer. During this period more than 20 Schools Loco's were based at Bricklayers Arms for mixed duties on the South Eastern and Central Sections and were used daily on Charing Cross/Cannon Street/London Bridge-Folkestone/Dover/Deal/Walmer/Margate trains which would normally consist of anything between 8 and 12 coaches. I had assumed it was a train for Folkestone Harbour but these did not call at the station but went directly to the yard where the tank locos were attached at the rear for the trip down the steeply graded harbour branch.
Brian gregory adds 'With that headcode and duty number I'd say it's a Charing Cross-Margate commuter train which could be an 11 coach load on some trains', thanks to Brian for all this info.