Prime Minister Tony Blair came under pressure in the Commons on Wednesday to call a public inquiry into the Potters Bar rail crash.
Relatives of the victims recently marked one year since the crash
Seven people were killed and more than 70 injured on 10 May 2002 when a West Anglia Great Northern train coach heading for King's Lynn in Norfolk became derailed 150 yards south of the station.
But he said an investigation into the incident would have to be completed before any decision about a further inquiry.
James Clappison, the Tory MP from Hertsmere, said that just over a year after the accident, the bereaved and injured had still not been told how it happened or who was to blame.
Mr Clappison asked: "Will you and your colleagues give serious consideration to the call from the families for a public inquiry?"
Mr Blair said he understood why it was particularly difficult "when so long a time has passed".
He said: "The Health and Safety Executive have said that they want to conduct the most thorough investigation.
"They haven't completed that yet.
"It is not for us to tell them when to complete it, but as soon as they do complete it we will analyse the outcome, and announce any further proposals at that time."