A 3,000-metre tunnel at an angle of 45 degrees would suck up heat and smoke as efficiently as a lighted cigarette.
Almost anything - electrical short circuit, mechanical friction, dropped cigarette - can start a flame, perhaps in plastic, or foam seating, or discarded wrapping paper.
After a few seconds of smouldering, the fire takes hold. Once a blaze starts consuming oxygen, it creates a vacuum that sets more air rushing towards it, literally to fan itself from a small flame into a big one.
Confined fires always burn more furiously than unconfined fires and a tunnel in which flame rises, drawing oxygen at an accelerating rate from below, is a perfect example of a confined space.
Machines are built of steel and alloy but there are parts of them that will burn if there is enough heat. Human flesh will start to suffer mild injury after one second at 65 C but fires generate 10 times that - and go on getting hotter as oxygen supplies start to roar in.
Heat alone will ignite flammable materials and set them blazing at 600 C. Fires under such circumstances accelerate with amazing speeds.
In the Mont Blanc blaze of 1999, a truck carrying flour and margarine became a ball of fire within 30 seconds. Temperatures reached more than 1,800 C.
At such temperatures, metals buckle and melt, stone chars and anything else bursts into flame. Polymers burn furiously at around 1,200 C, releasing clouds of smoke which are rich in combustible gases - and poisonous ones such as hydrogen cyanide. But the main killer in such cases is carbon monoxide, which can combine with the oxygen carrying haemoglobin in the blood 200 times faster than oxygen itself can do. Even small quantities of such gases can begin to stupefy victims.
People who are desperately trying to scramble away from a flame need more oxygen than normal, not less: oxygen makes up 21 per cent of normal air, and when the oxygen level falls to 15 per cent, judgment begins to deteriorate. At 6 per cent, hearts begin to fail.
Prof Bill McGuire is professor of geohazards at University College London and has travelled through the Kitzsteinhorn tunnel to the Kaprun glacier.
"This is the `in' way of getting up the Alps these days," he said. "A lot of cable cars are being replaced by these things: they plunge through the mountains and take people up to 7,000 feet in a couple of minutes. One of the big problems is, I can't see where people can go if the train stops and starts burning," he said yesterday. "The tunnel is so close to the train that getting out would be incredibly difficult."