Castle class 4-6-0 No 4090 Dorchester Castle inside Swindon roundhouse on April 7, 1957.

Peter Kerslake says "It was during the month of this photograph, namely April of 1957, that 4090 "Dorchester Castle" seen here iin Swindon shed's Roundhouse, received this double chimney, being the first "Castle" to be so equipped following lengthy trials from Swindon Works with 7018 "Drysllwyn Castle".
The locomotive is clearly in new colours and fresh from her overhaul in the adjacent Works and is doubtless about to be sent back to her home Depot at Old Oak Common following local "Running In" turns from here at Swindon shed.
Later years took 4090 to Swansea's Landore, Carmarthen, Neath, Shrewsbury and finally Cardiff East Dock after Canton had closed in September of 1962 and before she was condemned in June of 1963.
Even the earliest "Castles" enjoyed an extended lifespan follwing improvements and overhauls carried out in their late years by Swindon and 4090 has her mechanical lubricator positioned ahead of the main steam pipe, another refinement which kept them on Top Link express workings for a number of years."

R.O. Tuck/Rail Archive Stephenson