The Rail, Maritime and Transport (RMT) union is seeking a judicial review of the sell-off, partly on the basis that it allegedly infringes a European directive insisting that the state must stay in control of railway infrastructure. Railtrack owns the stations, track and signalling.
The union also believes that because the Railway Act 1993 contains no reference to the company, ministers have no power to sell its shares to the private sector.
The RMT will also argue at the hearing, expected early in the new year, that MPs were misled by ministers when the Bill was passing through Westminster.
Jimmy Knapp, leader of the union, says the Government always claimed that the sale of Railtrack would be "the guard's van of privatisation rather than the locomotive". Ministers, however, have since announced that it will be one of the first British Railsubsidiaries to be sold off, possibly in the first quarter of 1996.
Mr Knapp said the Government was acting outside its powers by hiving off the company. "Parliament has never had an opportunity to vote on the privatisation of Railtrack. The future of one of the nation's vital transport arteries should not be decided simply on the grounds that a desperate government wants to get its grubby hands on revenue to finance tax cuts." He said vital constitutional issues were involved.
A spokesman for the Department of Transport said ministers were sure they had "adequate and appropriate" powers under the Act and that government representatives had made no secret of their intention to sell Railtrack, in hundreds of hours of debate.
He said the sale of the company was essential to the whole privatisation process, and added that the union had not yet finalised the details of its argument.