GWR Castle 4-6-0 No 5017 formally St.Donats Castle, now The Gloucester Regiment 28th 61st (Egypt), with a Gloucester/Cheltenham-Paddington express recorded as approaching Iver on 6 July 1957.
Keith Morris says "Not convinced this is approaching Iver - there is no up goods loop - a feature between Langley, Iver and West Drayton. I would have though further west - Twyford to Maidenhead?", but other photos confirm it is indeed Iver.

Photo: K.L. Cook / Rail Archive Stephenson