I do not know much about GWR Broad Gauge but views of moving trains are very scarce. I think this is a ‘Swindon’ Class broad gauge 0-6-0 on a fish train - the last two vehicles look like Tadpoles with central caboose compartment and the first is a 6 wheeled 4 plank open used for fish traffic but now confirmed as Uffington, heading westbound with empty fish and meat waggons.

Photo: Rail Archive Stephenson