Police will be interviewing people involved in the crash.

Health and Safety Executive officials will be looking at the technical evidence, and will be drawing on Railtrack's records of signalling. Deputy Prime Minister John Prescott has called for a report from the Rail Inspectorate.

While the investigations take place, some familiar questions are sure to be raised.

John Prescott: "There will be a public inquiry"

The possibility of human error is bound to be considered. But attention will also inevitably turn to the reliability of signalling - a thorny issue in recent years following in particular the 1988 Clapham and 1997 Southall rail crashes.

Some reports said that Great Western and Thames trains were involved in a glancing blow. If that turns out to the be case, there may be echoes of the Southall crash which happened on the same stretch of track.

This would be particularly on the minds of people involved in the inquiry into the Southall crash, which is due to restart next week.

Early warning systems

Trains are currently fitted with an automatic warning system (AWS), which triggers alarms and flashing lights in driver cabs if red signals are passed.

Southall, 1997. Seven died

The inquiry into the Clapham crash - in which 35 people died - recommended the installation of automatic train protection (ATP) for the whole rail network.

ATP links the train's brakes automatically to the signals, and is similar to the system currently used on the London Underground; there have been doubts about its suitability for high-speed trains, but it has been tested on the Chiltern and Great Western routes and is used on Heathrow Express trains which run between Paddington and Heathrow airport.

But the �750m price tag for nationwide implementation was considered too high by British Rail and the government, immediately prior to privatisation in 1993.

It was calculated that although a full ATP system would save lives, it would be at a cost of �14m per life saved. The estimated cost of installing ATP across the network has now risen to more than �1bn.

However, the government announced this summer that a new train protection warning system (TPWS) would be fitted across the country by 2004. At �150m it is far more affordable than the ATP system.