Dr Iain West and Dr Ian Hill are working in 30-minute shifts, lying face down on platforms suspended inches above the knee-high debris in the blackened shell of coach H of Great Western's Cheltenham to London express.

Police say at least 40 people died in Tuesday morning's collision and fire, which happened when the express ploughed into a commuter service run by Thames Trains which had passed through a red light.

The BBC's George Eykyn: "No worker is being allowed to spend more than half an hour inside the carriage at a time"

Thirty bodies - 21 of which have been identified - have been recovered from the crash site near Paddington Station in west London. A further 64 people who may have caught the train are reported missing.

It has taken more than four days for coach H to be made safe enough to be searched by Dr West and Dr Hill, who will spend two or three days working at the site with a team of specialist Metropolitan Police officers.

Superintendent Tony Thompson, of British Transport Police, said: "It is everything you could expect of a scene that was affected by a fireball with an estimated temperature of 1,000 degrees...but some items of jewellery which could help identify the deceased may have survived."

A canopy has been placed over the carriage

The black box recorder of the Thames train has now been found. Supt Thompson said the find was a significant development and could bring key details of the crash to light.

Deputy Prime Minister John Prescott has ordered the Health and Safety Commission to begin an immediate investigation of track and signal operator Railtrack's role in the safety of Britain's rail network.

He made the announcement after he and senior rail industry figures discussed the crash with Prime Minister Tony Blair on Saturday.

The BBC's Peter Hunt reports on the government's plan to investigate Railtrack's role in safety matters

It has emerged that some carriages on the Great Western service were part of the train that crashed at Southall in 1997, killing seven people.

"Some of the coaches were the same as those involved in the Southall crash," said a Great Western spokesman. "After Southall, some carriages with only light damage were deemed repairable, subjected to a full safety assessment and reintroduced into service."

Click here to see the site of the crash

Sports fans across the country have held a minute's silence for the victims.

Recovering the victims is proving to be a slow, exhausting task

Among the crowds who fell silent were rugby fans at the World Cup clash between England and New Zealand at Twickenham, which two of the victims had been planning to attend.

An interim report by the Health and Safety Executive has said that the immediate cause of the crash appeared to be the Thames train jumping a red light, although it also highlighted failures within the UK rail system.

It ordered Railtrack to make improvements to 22 signals around the country which train drivers have repeatedly passed at red.

A second inquest opened and adjourned at Westminster Coroner's Court on Saturday named a further 10 victims of the crash.

They are:

• Derek Antonowitz, 25, of Willesden Green, north-west London
• Antonio Lacovara, 24, of Lewisham, south-east London
• Fela Ladipo, 33, of Muswell Hill, north London
• Neil Dowse, 39, of Forest Hill, east London
• Ola Bratlie, 26, of Norway
• Andrew Thompson, 52, of Lexden, Essex
• Dr Khawar Tauheed, 44, of Romford, Essex
• Bryan Tompson, 61, of Cirencester, Gloucestershire
• Alan Stewart, 28, of Fulham, south-west London
• Jennifer Carmichael, 22, of Thatcham, Berkshire

The emergency police number is 0171 834 7777. Worried relatives and friends should call the same number.

All people on the train or witnesses nearby are also asked to call the British Transport Police answerphone on 0800 405040 and leave their name and numbers so they can be contacted as part of the inquiry.

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