Thames Trains has pleaded guilty to breaches of health and safety law over the Paddington crash in which 31 died.
The crash at Ladbroke Grove, near Paddington, killed 31 people
It admitted two charges at London's Bow Street Magistrates' Court - one of breaching employee safety and the other relating to passenger safety.
A train going through a red light at Ladbroke Grove in west London on October 5, 1999, led to a crash with a First Great Western Train.
The case has been sent to the Old Bailey for sentencing.
The prosecution is being brought by the Health and Safety Executive under two sections of the Health and Safety at Work Act 1974.
The court has the power to issue an unlimited fine.
The Health and Safety at Work Act
Section 2(1) states: It shall be the duty of every employer to
ensure, so far as is reasonably practicable, the health, safety and welfare at
work of all his employees
Section 3(1) of the Act states: It shall be the duty of every employer to conduct his undertaking in such a way as to ensure, so far as is reasonably practicable, that persons not in his employment who be affected thereby are not thereby exposed to risks to their health or safety
Great Western Trains Company Ltd was fined £1.5 million after the Crown Prosecution Service brought a prosecution under the Act following the 1997 Southall rail crash in west London in which seven people died.
The HSE announced in March 2002 that it intended to prosecute both Thames Trains Limited and Railtrack (now Network Rail).
The way was cleared in August for the organisation to proceed with the case after both the British Transport Police and Crown Prosecution Service said there was no prospect of a prosecution of Thames Trains for manslaughter.
A group of Paddington survivors is still fighting for compensation claims, although one - Pam Warren - received a "substantial sum" in October.