What can that be in the freight going the other way?

Hauled by the customary Gloucester allocated " Castle " , on this day 5042 " Winchester Castle " , the Paddington bound " Cheltenham Spa Express" runs non stop through Swindon station and provides the fortunate photographer a fine sight to capture on film.
I believe that 5042 was a Gloucester engine for the whole of its thirty years in service , and I recall that it became one of the very last handful of " Castles " to remain in use before its withdrawal from that shed in June 1965 .
Immortalised by driver J.W.Street in his book , " I drove the Cheltenham Flyer " this express working between Gloucester and Paddington achieved fame when 5006 , " Tregenna Castle " set a record time on a London bound working , but tales of fast and furious runs on the " Flyer " in days past were common in those happy times.