LMS Fairburn 4P 2-6-4T No 42104 with the 4.20 London Bridge-Brighton near East Grinstead on 23 May 1955.

lan Goodwin suggests "I think it might be coming out of the cutting that the Bluebell Railway had to re-excavate to reach East Grinstead, Imberhorne Lane Bridge being just visible".
Upon investigation the lay of the track is perfect and when I zoom in on the tiny part of the bridge the brickwork pattern with its recessed panels is identical.

Photo: J. Head / Rail Archive Stephenson