Lord Kirkwood ruled in Edinburgh's Court of Session that BR's decision to end the Fort William sleeper service on 26 May was unlawful because it provided the only trains on sections of line in the Glasgow area.
BR intended to run regular "ghost" trains late at night on the lines to ensure that technically they were still open. However, Lord Kirkwood accepted the argument by the Highland Regional Council, which mounted the challenge, that this was a "sham" and managers had failed to follow statutory line-closure procedures.
It was the first challenge to a closure under the Railways Act 1993, the legislation for privatising rail services, and its success suggests any other attempts to reduce services by BR or new private operators will be similarly challenged, adding to the already considerable uncertainty over rail privatisation.
If Lord Kirkwood's decision is upheld in the higher courts, it will mean that the proposal will have to be submitted to full public consultation, which takes about a year. In the meantime, the service must continue, he said, because the "ghost" trains did not amount to a service.
The plan to cut the service, along with Motorail services and the sleeper to Carlisle, emerged last December when Roger Salmon, the franchising director for the railways, announced that he would not be providing subsidy for these services when he took responsibility for allocating government support for rail services.
Mr Salmon said yesterday that the closure of the service was a matter for the BR board and Scotrail.
BR had argued it would have saved more than £2.5m a year by cutting the overnight service on the West Highland line but the line's supporters have said that the subsidy is much less.
Scotrail, which runs BR's Scottish service, says it will be plunged into a financial crisis if it has to operate the service without being bailed out by the British Rail board. Last night, BR said it would appeal but admitted it would be taking reservations for the service for journeys after 26 May.
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