Network Rail and the rail maintenance company Jarvis have accepted legal responsibility for claims brought over the Potters Bar rail crash.
Seven people died in the Potters Bar rail crash in May 2002
In a joint announcement the two firms said accepting liability would provide "comfort and assistance" to victims.
A Jarvis spokesman added the move did not mean the firm was to blame for the accident, which killed seven people.
Seventy-six people were also injured when a train derailed at Potters Bar station in Hertfordshire in 2002.
The crash was caused by a faulty set of points, operated by Railtrack (now Network Rail), and maintained by engineering firm Jarvis.
A Network Rail spokesman said: "Network Rail and Jarvis hope that by formalising the liability issue, those
affected by the tragedy will gain some level of comfort and assistance.
"In the meantime, the industry parties will continue to work with the Health and Safety Executive as
its investigation continues."
Network Rail had until now taken sole responsibility for claims, but this announcement means Jarvis will pay half the compensation bill, which could reach £3m.
A Jarvis spokesman told BBC News Online: "What we are not doing is accepting liability for the cause of the accident. The fundamental cause remains under investigation."
The Hertfordshire-based firm said at the time that vandalism could have been to blame, but subsequent accident reports found no evidence of this.
The victims and bereaved have been angry that no company had until now formally admitted liability.
Writer Nina Bawden, 77, whose husband Austen Kark, 75, was killed in the crash, said: "It's a great relief that liability has been admitted, but it does not diminish the need for a public inquiry."
Ms Bawden, who was badly injured in the crash and had to spend two months in hospital, said that nearly two years after the accident she was still having trouble walking.
Bob Crow, general secretary of the Rail Maritime and Transport Union, said: "This is a nonsense. How can they accept liability without accepting
A police inquiry into the crash has been sent to the Health & Safety Executive for further investigation.
A spokeswoman for the HSE said: "We have produced three interim reports on the crash and a full report will not be published until the end of any criminal investigation.
"Our work continues into the root cause of this accident."
An interim report last year said poor maintenance led to
the points failure which caused the West Anglia
Great Northern passenger train to derail on May 10, 2002.