I missed this episode, but I'm fairly certain the accident mentioned was in 1957. It wasn't a 'blowback' in the more usual sense of a superheater tube giving way and forcing steam and flame into the cab, but the failure of the feed to the steam brake valve - rendering the loco brakes useless and also filling the cab with quantities of steam, preventing the crew from operating the controls. Full details can be found in the report at
http://www.railwaysarchive.co.uk/documents/MoT_Chapel1957.pdf.
The acxcident and the bravery of the driver was the subject of a BBC Radio programme "The Ballad of John Axon" several years after the event.