Network Rail is facing an unlimited fine after admitting health and safety breaches over the 1999 Ladbroke Grove rail crash that claimed 31 lives.
The crash at Ladbroke Grove, near Paddington, killed 31 people
It admitted charges under the Health and Safety at Work Act 1974. Sentencing was adjourned to 18 December.
The crash happened when a Thames Trains train passed a red signal and hit a Great Western express in west London.
The plea is an admission of risk creation - not an acceptance of responsibility for the accident.
Relatives of three of those who died called the adjournment yet another example of "prevarication" by Network Rail.
After the 20-minute hearing at Blackfriars Crown Court in south London, Jonathan Duckworth, chairman of the Paddington Survivors Group, said: "We are pleased they have pleaded guilty, but the plea will reduce the amount of the fine."
The indictment against Network Rail, which owns and operates Britain's rail infrastructure and was Railtrack at the time of the crash, cited various breaches of the 1974 Health and Safety at Work Act.
Among them, it listed inadequate signal sighting distance, said part of a signal had been obscured by a large insulator, and criticised the signal's visibility.
It said the signal configuration at the scene had been "found nowhere else in the UK".
It further criticised Network Rail for failing to ensure a signal sighting committee met after six Spad (Signals Passed At Danger) incidents between 1996 and 1998.
And it said the company failed to conduct any "adequate risk assessment" or investigation following the Spads.
On 18 December, Network Rail will indicate the full basis of its guilty plea in writing.
Nigel Sweeney QC, defending, asked for the adjournment to allow time for a mass of "unused material" to be examined.
Depending on the result, the Crown may seek a special Newton hearing, to determine Network Rail's precise level of culpability.
A Network Rail spokesman said in a statement: "The Ladbroke Grove tragedy was a terrible event for everyone involved.
"Lessons have been learned and the rail industry has changed enormously for the better over the past seven years."
More than 400 people were injured in the crash on 5 October, 1999.
Thames Trains and Railtrack were both criticised in a subsequent public inquiry report by Lord Cullen.
Thames Trains pleaded guilty to health and safety offences in relation to the 1999 crash and was fined £2m in April 2004.
Meanwhile, the Office of Rail Regulation said the number of Spads this summer had risen compared with last summer.
There were 94 on Britain's mainline railways from July to September this year - eight more than in the same period last year, it said.
The ORR said the rise was "possibly connected with exceptionally high temperatures".