Investigation to focus on maintenance of electrical equipment on the train

One possible cause of yesterday's accident is poor maintenance of the train. This could have led to a piece of the current collecting equipment – a wooden block called a negative shoe beam – falling off, eventually causing the derailment of an axle.

One possible cause of yesterday's accident is poor maintenance of the train. This could have led to a piece of the current collecting equipment – a wooden block called a negative shoe beam – falling off, eventually causing the derailment of an axle.

Bob Crow, the leader of the RMT rail union, said last night that a driver had reported a fault at Leytonstone 15 minutes before the accident. Mr Crow alleged that a motor block which provides electricity to the train had fallen on the track.

Trains are supposed to be checked every morning before they leave the depot, and clearly that procedure will be examined by the accident investigators. If poor maintenance is found to be the cause, it will give rise to a host of doubts about the PPP, which the Government has pushed through in the teeth of almost universal opposition. Critics have included not only the Mayor of London, Ken Livingstone, but virtually all transport experts.

The accident could not have happened at a worse time for the Government. It came just four weeks after the signing of the first of the controversial contracts for the Tube under the public-private partnership and will raise questions about the way maintenance work on the system is organised.

Under the PPP, the infrastructure, including the trains, and its maintenance are contracted out to private companies under deals lasting 30 years. The operations – the day-to-day provision of train services – remains in the public sector. The Central Line, where yesterday's accident happened, was not included in the first PPP contract which related only to the Jubilee, Northern and Piccadilly lines. However, for the past three years its maintenance and provision of its train services have been separated out.

Trade unions and many rail managers have expressed concern that poor communication between the different parties may lead to safety risks. This has already happened on the national railway; the accidents at Hatfield and Potters Bar are both thought to have been the result of basic maintenance errors.

The PPP was conceived by Tony Blair's first Labour government when John Prescott was Transport Secretary. It brought in private finance without outright privatisation, which was vetoed in the 1997 Labour Party manifesto. However, to bring in the private sector the operations had to be separated from the infrastructure, and this has been at the root of much of the controversy because it mirrors what was done by the Tories to the national railway.

Mr Livingstone has argued throughout that the PPP was likely to result in a fatal accident. The Mayor was planning to announce tomorrow that he will continue pursuing his opposition to the PPP deal, even though one of the contracts has been signed, by appealing in Brussels against an earlier ruling that the contracts did not contravene European legislation. This incident is bound to strengthen his resolve in pursuing what has seemed a hopeless legal cause.

Christian Wolmar's book 'Down the Tube: the Battle for London's Underground' has just been published by Aurum Press, £9.99