British Transport Secretary Stephen Byers says a faulty switching mechanism is to blame for a passenger train derailment last week that killed seven people and injured 70 others. Two nuts were missing from the switching mechanism, he said, and that caused the fatal derailment Friday at Potters Bar, a suburb north of London.

Mr. Byers said health and safety inspectors are still examining the incident, and he will hold off on ordering a public inquiry until they produce a report.

This is the fifth fatal railway crash in Britain in the past six years, and it has raised new safety concerns among the traveling public. Still, Mr. Byers told parliament on Monday that rail remains one of the safest means of transport in Britain.

"If one looks at the long-term trend, then it is true to say that per passenger mile traveled, railway travel is getting safer," he said.

In a separate development, a union leader says a railway worker warned management three weeks ago of track maintenance problems near the scene of the fatal derailment.

Bob Crow, chief of the Rail, Maritime and Transport Union, said Railtrack should stop using private maintenance contractors.

"What's basically happened here is that you've got something in the vicinity of 3,000 agencies working on the railway network, eight infrastructure companies, all competing against each other," Mr. Crow said. "And all trying to make a profit. And what Railtrack should be doing is having all of this work brought back in house."

Railtrack chief John Armitt admits he has been concerned about poor maintenance standards by unqualified workers, even before Friday's crash. He told The Times newspaper that short cuts and cursory inspections by unskilled maintenance crews are problems that must be fixed.