The Big Question: Why are Channel Tunnel repairs taking so long and is it still vulnerable?

Why are we asking this now?

Yesterday, the Channel Tunnel operator Eurotunnel signed a deal with a consortium of construction companies to repair the damage caused when a lorry caught fire near the French entrance last month. The fire closed the tunnel between Kent and Calais for two days and there has been a reduced rail service ever since. Repairs are expected to cost £47m and be completed by February – seven months after the blaze.

What happened inside the tunnel?

On 11 September, a lorry shuttle train was carrying 17 vehicles from France to England. It was about seven miles into the tunnel, just over a fifth of the way through, when automatic smoke detectors warned that one of the lorries was on fire. Investigators still do not know what caused the blaze but they suspect the vehicle's braking system overheated and set one of its tyres alight. There were 32 people aboard the train at the time and all were quickly ferried to safety. Most were lorry drivers, travelling in a club car at the front of the train. When the club car and locomotive were removed from the damaged tunnel earlier this month, Eurotunnel made sure there were witnesses on hand to determine that the fire had not spread quickly enough to do any damage to either – thereby emphasising that neither the lorry drivers nor the train crew were in any real danger during the incident.

Why is it taking so long to repair?

Work could not start with the damaged lorry shuttle still in the tunnel. It could not be removed because, as soon as the accident occurred, the judicial authorities took charge and Eurotunnel was not allowed to move anything without their consent. Officials wanted to know whether the automatic alarm systems worked, how quickly the fire spread, whether passengers or crew were in danger and whether the accident was ever likely to escalate into catastrophe. They have now allowed the debris to be cleared away. Also, before repairs to the tunnel could begin, Eurotunnel had to install a site office and other accommodation for the workmen, to minimise the amount of time they will have to spend travelling the 11 miles from the tunnel entrance to the scene of the fire.

The quickest way would be for them to use the service tunnel, but that has to be kept clear to ensure passengers have a safe escape route if another fire breaks out. The office and accommodation are now ready, and Eurotunnel seemed confident yesterday that the job would be finished in four months.

Has the accident stopped people using the high-speed link to Paris?

With the tunnel being closed for two days, you might expect the latest figures issued by Eurotunnel and Eurostar, covering the three months from June to August, to show a dip in freight movements and rail passenger numbers. Eurotunnel calculated that the fire cost it £22m in lost revenue in the second half of September alone. Without the fire, it said, its income would have gone up 5 per cent. Instead, it fell by 6 per cent. However, the two companies say their losses are covered by insurance. Yesterday, Eurostar reported that its ticket sales over the third quarter were £152.3m – or 2.4 per cent higher than the same period a year ago. The number of passengers carried rose 6.4 per cent to 2.27 million, despite the fire. Meanwhile, ferry operators, who have been having a tough time staying competitive since the tunnel opened to rail traffic in 1994, have enjoyed a rush of extra business since September. One way or other, people are as determined as ever to get across the Channel.

So, you are in the tunnel and your train is on fire – how do you get out?

We call it the Channel Tunnel when actually is not one tunnel, but three – one each for northbound and southbound trains, with a service tunnel in between. There is a 100ft gap between the two main tunnels, so a fire or explosion on a train will not affect a train heading the opposite way. The service tunnel is only 15ft wide, so it is also a safe distance from the tunnels on either side. The air pressure in the service shaft is higher than in either of the other two tunnels, which prevents smoke from seeping in. If a train has to come to an emergency halt, passengers can disembark on to a walkway and make their way to the nearest cross passage to the service tunnel, which is never more than 205 yards away. Specially-designed shuttle cars can travel along the walkways, so the passengers will not have to walk the full distance back to land.

How vulnerable are travellers in the tunnel?

In some respects, it is safer travelling under the Channel than on a normal railway line. Down there, there is no danger from those occasional overground hazards such as falling trees, children throwing rocks or drunken motorists veering on to the line. And if a train derails beneath the Channel, there is no risk of it being hit by another coming the other way. However, like the London Underground, the tunnel is a tempting target for a terrorist attack, and one thing we do not know is what it might take to cause the roof to cave in. Trains go through tunnels under cities and through mountains all over the world, and the tunnel roofs do not fall on them – but there again, no other tunnel goes under a stretch of water as wide and deep as the Channel. These tunnels are more than 30 miles long, and nearly 24 miles of their length is below the sea. People are now asking whether there ought to be stricter rules about using them to transport any form of flammable material.

Earlier this week, the Labour peer Lord Berkeley of Cranford, a trained civil engineer and former public affairs manager for Eurotunnel, told the House of Lords: "This is the second major fire that has taken place since the tunnel opened and, in both cases I believe, a very large length of tunnel lining has burnt out and collapsed – which of course means that the sea bed is standing up on its own."

What are the British and French governments going to do?

They have done what governments always do when they are faced with an unexpected problem – they have set up an inquiry. The Channel Tunnel Safety Authority has been instructed to "conduct a fundamental review of the extent to which experience has modified the original risk-assessment assumptions and to make recommendations". The new Transport Minister, Lord Andrew Adonis, has said he is expecting the authority's report by September next year at the latest.

Has the fire had any knock-on effects?

It has been no fun at all driving on the motorways in Kent. Port control officers went on 24-hour strike in Calais and Boulogne yesterday, which meant that lorries could not cross the Channel by ferry, while their chances of getting aboard a truck shuttle were limited by the fire damage. A long stretch of the M20 turned into a lorry park. Kent police have now warned that the combination of disruption in French ports and the damaged tunnel will cause a "potentially dire situation" on the roads.

Should we travel by tunnel?

Yes...

* Train accidents, although serious when they happen, are very rare. You are safer in a train than a car

* Eurostar is amazingly quick. You reach the centre of Paris less than three hours after you leave London

* Moving the main international terminus to St Pancras has shortened the trip from anywhere north of London.

No...

* If a small fire that hurts no one disrupts services for five months, think what a major incident would do

* Incidents are rare, but very frightening when they happen because a tunnel is such a confined space

* Travelling underground is boring. A ferry is twice as much fun, especially if you're travelling with children

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