More Central line misery

Commuters face many more weeks of misery because Tube chiefs still do not know what caused the Central line derailment.

The problems are so bad that London Underground admitted today it could not guarantee even a limited service when trains are allowed to run again.

And no date can yet be given for when a full service will resume on a line that was used by more than 650,000 passengers a day.

An interim report published today fails to reach a definite conclusion into the causes of the accident at Chancery Lane station on 25 January which injured 32 passengers.

The crash closed both the Central and Waterloo and City lines on safety grounds.

As a result, LU's director of safety, Mike Strzelecki, said that while it was "hoped" to get the full number of trains running again on the Central line by the end of next month, he admitted: "I cannot guarantee it. I cannot give a date when that will happen."

The report confirms that a half-ton undercarriage motor broke free from its four main holding bolts, derailing the last four carriages of the eight car train.

LU says metal fatigue is the most likely cause of the bolt failure.

But their investigations continue into that and other possible failings in the traction drive systems or the gearboxes. The report reveals a dramatic list of failings but does not identify the overall cause of the accident. It says:

* Vibration could have caused the motor mountings to wear.

* There were a "series of failures" in the motor's traction drive mechanism before it became detached.

* There are "indications" of gearbox failures.

* The "black box" data recorder of the crash train provided evidence that something may have been wrong with a motor - but was not available to the train driver or line controller at the time.

* Investigators are studying similar problems of motors dropping off from mainline Networker London commuter trains in the late 1990s.

All 85 Central line trains are still undergoing safety checks but engineers have only just begun to replace 11,200 bolts and 5,600 safety brackets on 2,800 motors.

LU is spending most of the time instigating a new "quality control" inspection regime to ensure that all bolts and safety brackets are fitted correctly and to the required tensions.

LU was forced to drastically increase the inspection regime following the fiasco on the Waterloo & City line - which uses the same stock - earlier this week.

It was found that trains were allowed to start running again without the newly fitted bolts holding some safety brackets in place being tightened properly.

The bolts are understood to have been inserted only finger-tight. It is reported that senior managers "went ballistic" when it was realised that a special team given the job of securing the bolts to the correct torque had failed to do the job properly.

This could delay the start of the Central line even more while the whole bolt checking process is reassessed.

Mr Strzelecki said: "We apologise to passengers for the extra inconvenience that this will cause. But it is more important to make sure we get this right so that it is safe rather than rushing it and getting it wrong."