Thanks to Roman Schmidt who confirms the location. A large yard in open countryside, the location is Moreton sidings, east of Didcot this was a yard on the Up side.

Peter Kerslake says "This is a "down" working heading westwards and approaching Didcot.
Headed by a Welsh based engine at this date this is undoubtably a working from Paddington bypassing Bristol Temple Meads and bound via Patchway and the Severn Tunnel to Cardiff and possibly further westwards to Swansea and even beyond.
5037 "Monmouth Castle" was the engine portrayed by the artist Terence Cuneo in his "Clear Road Ahead" painting, his primary sketches of the footplate and its crew requiring the artist to be perched aloft on the coal within the engine's tender whilst they headed an express working, not the most comfortable of locations !
After spells at Neath and Llanelly in the early 'sixties "Monmouth Castle" went to Bristol St. Philips Marsh in October of 1963 where it remained until withdrawal in March of 1964, the former passenger locomotive depot at Bath Road having earlier closed in September of 1960."