By Tom Symonds BBC Transport Correspondent

Passengers have been killed at Watford, Southall, Hatfield and Potters Bar.

But the disaster at Ladbroke Grove, where the railway approaches London's Paddington Station, was a level above.

The two trains collided at a combined speed of 130 mph.

The disaster horrified the nation

Some were killed by the impact, others by a series of fireballs that swept through the wreckage.

In all 31 died, and more than 400 were injured.

Almost 150 people were travelling in just three carriages of the Thames Trains service - only six escaped injury.

The train driver was killed.

Michael Hodder had only qualified 13 days earlier, but he was negotiating some of Britain's most complex railway lines.

The poor standard of training he was given, has left his employers with a bill for �2 million.

Red stop signal

The immediate cause of the crash was the failure of Mr Hodder to notice a red stop signal - SN 109 - as he left Paddington.

He drove on into the path of a high speed train.

But signal 109 had been passed 8 times by other drivers while set to red.

They'd reported it as difficult to see among the gantries, cables and bridges on the line.

At the start of 1999 Thames Trains began to revamp its driver training programmes to give staff information about signals which had posed problems in the past.

But the changes hadn't been fully introduced.

Poor training

Michael Hodder wasn't sent on an awareness day for extra signal training.

He wasn't given adequate instruction on the complicated Paddington approaches.

Route maps weren't kept up to date.

Even instructors weren't aware which signals on the line had a history of being missed by drivers.

Thames Trains has always admitted its role in the tragedy.

Record fine

As a result the huge �2m fine was lower than it could have been - the judge, Mr Justice Rodger Bell reduced it by a third because Thames pleaded guilty.

But it does set a new record for penalties extracted under the Health and Safety at Work Act.

It exceeds the �1.7m fine handed down to four companies involved in the collapse of a walkway at Ramsgate Harbour in 1994.

It is higher than the �1.2m paid by Balfour Beatty after a tunnel being constructed at Heathrow Airport collapsed.

It is also more than the �1.5m paid by Great Western trains following the Southall crash in 1997.

Until last week the Thames Trains service was operated by the Go-Ahead Group, who will have to pay the fine.

But now the service is being run by another big transport operator FirstGroup, which has taken over with a promise of more frequent trains, and of course, the highest of safety standards.

Lessons

So have lessons been learnt?

Its four-and-a-half years since the accident, and the rail industry has spent much of the intervening time examining the Paddington crash.

There was a mammoth public inquiry chaired by Lord Cullen.

Driver training has been revamped, with Thames Trains, its experience still raw, leading the way.

Network Rail has recently finished installing a new warning system that stops many trains passing red signals.

Since the work was completed across Britain there hasn't been a major train accident.

Rail chiefs dare to hope that there'll never be another Paddington crash.