The "severe" fire which engulfed an entire carriage in the west London rail crash is the worst to have happened on modern trains, according to experts.

Professor of transport safety at University College London, Andrew Evans, told BBC News Online that one of the most important questions which must now be asked is whether fires of such intensity could in future be prevented or better controlled.

He said: "One of the most important tasks for the inquiries must be to determine how the fire occurred.

"There have been fires on trains before, but there has never been such a serious fire, or one with such terrible consequences, in the aftermath of a crash."

Editor of the Railway Gazette magazine, Murray Hughes, told BBC News Online that diesel fuel carried in the Intercity 125's powercar would have escaped upon impact.

He said: "The fuel would probably have leaked over a large area quite quickly, some of it vaporising. Any spark would have been enough to ignite it - and there would have been plenty when the powercar landed back on the ground.

"That would have accounted for the huge explosion that was reported almost immediately after the crash.

"But you have to remember that diesel does not ignite as readily as for example, petrol, and is a commonly used fuel all over the world."

The fire was "almost certainly" the cause of many fatalities

London Fire and Rescue spokesman Mark Bloomfield described the fire as "severe" to reporters, saying that temperatures were high enough to weld metal to metal.

Mr Hughes said that Intercity trains carry enough fuel to make several journeys, and can hold up to 5,500 litres of diesel.

"The only similar incident to this fire in recent times was when an Intercity 125 train had a fuel tank fall off, but that fire - in the mid 1990s in Maidenstone - did not have the same terrible consequences as this one."

Prof Evans said that although the focus of safety concerns in the immediate aftermath of the incident has been signals and driver response to them, the fire was something that needs to be thoroughly investigated.

He said: "Until Tuesday, the worst accident with modern rolling stock had 13 fatalities, and that was in Polmont in Scotland in 1984.

"The Ladbroke Grove accident is by far the worst accident ever to have occurred to modern trains.

The heat was sufficient to weld metal to metal

"The reason for the high death toll is the intensity and indeed the immediacy of the fire.

"This accident will change our faith in modern rolling stock. It wasn't previously thought that the consequences of a collision involving modern trains could be so bad. Now we have to see that the consequences can be fairly appalling."

He said that the rail crashes of the past which had very high death tolls - Clapham in 1988, which claimed 35 lives, and Hither Green in 1967, in which 49 died - all involved old trains.

He said: "It is shocking that so many people will have died on rolling stock that was specifically designed with safety in mind."

Rescuers have still not been able to recover all bodies from the wreckage

Mr Hughes, however, said that he doubted any serious changes to the way in which fuel is carried on trains will be effected as a result of the crash.

He said: "Trains need to carry fuel, and this type of incident is so rare that any theoretical amount of money spent devising ways to make fuel carriage safer would probably be better used in other areas.

"If for example, you increased the price of tickets to cover this cost, you could discourage people from travelling by train, and encourage them to travel by road which is a great deal more dangerous anyway.

"Making sure that collisions don't occur in the first place has to be the straightforward way of improving safety on the rail network."

The vast majority of fires on trains - there were 344 reported in the 1997-98 period - are the result of vandalism and arson.

During that period, 315 fires reported happened on passenger trains, and of those, 65% were the result of arson.