The families of five people killed in the Potters Bar rail crash are about to launch legal action to find out who is legally responsible for their deaths.

As the six-month anniversary of the tragedy approaches on Sunday, there is still no end in sight to the official investigation into the crash, which killed seven people and injured 76. It is established that missing nuts on an adjustable stretcher bar on a set of points near the station caused the crash, but it is not known why they were missing, or who was responsible.

Five of the seven bereaved families, and 14 of those who were injured, have decided they cannot wait any longer for the official inquiry to be completed, and are now taking matters into their own hands.

Louise Christian, the solicitor representing the families, said the action would be directed against Railtrack and Jarvis, the firm responsible for maintaining the track at Potters Bar.

"We're about to issue proceedings in relation to liability because nobody admitted liability," she said.

"We are going to ask the court to determine who's liable. The failure to admit liability draws out the distress and trauma of the victims and that is what's particularly unacceptable."

She pointed out that the official inquiry into the Hatfield crash is still continuing more than two years later and says there is no reason to believe the Potters Bar investigation will end soon.

The safety of the railways and the for-profit track maintenance has been called into question, yet little official action will be taken until the inquiry by British Transport Police and the Health & Safety Executive has concluded. Until then, neither Railtrack nor Jarvis will accept responsibility and crash victims and traders whose businesses were affected by the crash will continue to suffer without compensation.

Best-selling novelist Nina Bawden, 77, was on board the train when it crashed. Her husband Austen Kark was killed and she is still suffering from post-traumatic stress disorder and physical injuries.

In an interview this week, she said: "I would like them to come clean and say 'we didn't run this railway properly, it is our fault that the tracks were defective and we're sorry'.

"And obviously there should be compensation for people, because it should be made clear to the railways that it's actually more expensive to kill people than to keep the railways safe."