As Clifford Forsythe, the Ulster Unionists' transport spokesman, signalled that his party's nine MPs were hostile to privatisation, the Northern Ireland Office moved to reassure them that there was "no truth" in reports that railways in the province would be sold.
With all the other minor parties also opposed to the controversial privatisation, tonight's vote could turn on the loyalty of just one Conservative MP.
Mr Forsythe, MP for South Antrim, warned that the railways must remain an integrated public service. He said: "Rather than having a disjointed system, the roads and railways in the whole of the UK all should fall into one plan. Essential rail links should be retained as a public service rather than simply on whether they are financially viable."
The people of Northern Ireland had an interest in Railtrack, because of the importance of the West Coast line from London to ferry points on the Irish Sea.
But the Northern Ireland Office statement faxed yesterday to Unionist MPs appeared to satisfy their concerns about the future of Northern Ireland Railways.
The statement repeated the Government's commitment to "a programme of maximising private sector expertise" through market testing and contracting out, but said there was "no current timetable for privatising public transport in Northern Ireland". Fresh legislation would be required for privatisation in the province.
With the voting intentions of the Ulster Unionists still unclear last night, potential Tory rebels denied that they would break ranks. A Tory party spokesman said he was confident of a government majority of about 20.
Sir Keith Speed, the MP for Ashford, said he was in close touch with the half-dozen Tories who lobbied Sir George Young, Secretary of State for Transport, just before Christmas. He said yesterday: "I would be very surprised if any of them would abstain, let alone go into the Opposition lobby on Wednesday."
Tony Blair, the Labour leader, upped the stakes in the Commons yesterday, when he raised the sale issue at Prime Minister's Question Time.
He asked if one reason for the Tory defeat in last week's Staffordshire South East by-election was not that "people don't want to see their railway system broken into a 100 different companies, sold on the cheap, laced with sweeteners out of the taxpayers' money?" He said "hundreds of millions of pounds" were being spent on selling-off the railways "when the vast majority of people would like to see that money spent on improving it as a public service".
Mr Major said previous Labour governments had closed more than 600 stations while the Tories had opened more than 220.
Under Labour, fares had risen by more than 20 per cent while the Government had pegged fare rises to inflation and fares would drop later.
Opposition to the sell-off among the public is underlined by an ICM poll for today's Guardian, which suggests 43 per cent think services will be "less safe" when the railways are fully privatised, 47 per cent think they will be "worse" and 79 per cent think they will be "more expensive".
Meanwhile, the Save Our Railways campaign went to the High Court in London yesterday to seeking leave for a judicial review of the franchising process over the refusal by the franchising director, Roger Salmon, to allow British Rail to bid for the London, Tilbury and Southend franchise.