In fact one of the most important fire disasters in England, the Kings Cross fire started because nicotine addicts used to light up on their way out of the tube (which had already prohibited smoking, it's kinda crazy to imagine people in the 1970s used to smoke on the fucking underground railway, deep under the ground where you are unavoidably breathing the same air as everybody else)

Somebody probably drops a spent match, it's still smouldering and it drops inside the escalator where it finds plenty of fuel and begins a fire. From there you mostly just need bad luck - yes the staff could be better trained, but even when they do summon professionals the firefighters don't arrive in time to tackle it while it's still small, the then-unknown trench effect allows the hot gases to pool and initiate flash over suddenly, a bunch of people die.