RAIB: "... The locomotive entered the tunnel, and the fire blew back, filling the cab with flames"

Link to RAIB
(which also allows you to discover what that is)

"At 11:04 hrs on Sunday 27 May 2012, train 1Z54, the 09:38 hrs special service from Finsbury Park to Rowsley operated by West Coast Railways (WCR), suffered a blowback of the locomotive fire as the train entered Wood Green tunnel in north London. 

"There were three staff on the footplate at the time, and all of them were injured by the flames. Two of them, the fireman and a member of the support crew, were taken to hospital for treatment to their burns."