60xx 1951c where
GWR King 4-6-0 No 60xx in c1951.
Train reporting number is 635 which was the 10.00am Penzance – Paddington' the Up Cornish Riviera Express.
Possibly east of Westbury around Lavington (but again the telegraph poles look incorrect)? To the left of the pole in the distance is the side of a hill or the edge of a scarp slope, which would be right for the northern edge of Salisbury Plain. There aren't many sharp bends to match the picture between Castle Cary and Newbury and west of Castle Cary is pretty flat.
Photo: W.J. Verden Anderson / Rail Archive Stephenson