The formidable looking building in the background is the prison.
Robert Waterman says "The date is 15th October 1966. The was a special from Waterloo to Westbury via Salisbury behind 35023 achieving 101/102mph at Andover and 98mph at Porton (8 coaches - 1/2 throttle, 25% cut-off as I recall - driver Bert Hooker, Nine Elms). The train reversed at Westbury and was taken forward by 35026 with a Salisbury crew via the GW route to Exeter St Davids (hence the standard class 1 train headcode displayed) then banked by a Warship up St Davids bank to Exeter Central as seen here. As there was a layover here, in order to clear the up main through platform for other traffic the whole train was drawn forward and then shunted back into the up bay (behind the concrete fence) and departed from there non-stop to Salisbury after the layover. If I recall correctly 35026 did not come off the train at Exeter. At Salisbury, 35023 took over again for the non-stop run to Waterloo".