The green lined 43xx is on humble freight duties. Note the pre-grouping GWR wooden post signal with unusual 'Calling On' arm.

Peter Kerslake says "My 1956 shed book shows 6361 as being a Pontypool Road engine at that time but by the time of this picture it was off Aberdare.
This is a westwards coal empties returning to South Wales in the via the Severn Tunnel.
The locomotive and her load are waiting on the Downthrough road here at Swindon station, and this location woud have been a fine spot to capture on film the Top Link passenger workings speeding through without stopping, especially those bound for Paddington such as the "Capitals United" and "Red Dragon" from South Wales in addition to the " Bristolian " and "Merchant Venturer" from Bristol and the "Cheltenham Spa Express" to name just a few."