GWR Castle 4-6-0 No 5099 Compton Castle on a down express in c1949, between Reading and Twyford. Woodley Bridge signal box is in the distance.
5099 entered traffic in May 1946; the loco would have been released into traffic in the Great Western's 'G-emblem-W' livery. The lettering on the tender is the short-lived BRITISH RAILWAYS (in Egyptian Serif font) so the photo is therefore no earlier than 1948. It had a Heavy General overhaul in August 1949 and another in June 1951 (at both times it changed its tender) so this will be between these dates.
Peter Kerslake says " 5099 could well be working back to its home at Cardiff following an earlier run up to Paddington this same day, the usual 'Out and back' roster for Cardiff engines and their crews incorporating a brief lay over at Ranelagh depot at Paddington, the engines having had sufficient coal at Canton for such a double turn".

Photo: Rail Archive Stephenson