Network Rail yesterday applied for an injunction to prevent a 24-hour stoppage by 7,500 signallers and maintenance workers, which is due to begin at 6pm on Tuesday in coordination with a walkout on the London Underground.
Senior executives say that the Rail, Maritime and Transport union sent voting papers to members working at signal boxes which do not exist, to station cleaners who do not work for the company, and to people listed as working at "unknown" locations, in con travention of employment law.
But Network Rail's legal challenge threatened to backfire by causing a further deterioration in relations with the RMT, which said it would hold an even bigger vote if its strike mandate was ruled invalid.
Union sources said a delay to the strike would only make matters worse by giving time for more maintenance staff to join Network Rail under a programme of bringing contractors in-house. It could lead to an even bigger vote for strike action by an enlarged workforce with hardening attitudes.
An RMT spokesman said: "The matter is in the hands of our legal team and we will vigorously defend our position."
RMT members backed a strike by 2,947 to 2,246 last month in a dispute centred on the closure of Network Rail's final salary pension scheme. In the biggest bargaining group, signallers, the majority was just 56. The walkout would cripple Britain's rail network with up to 90% of trains cancelled.
But Network Rail said 12 of the RMT's members were listed as working at a Brighton signal box which closed in 1983. Six were registered at a Newcastle site which shut 14 years ago and one was said to work at Garston signal box, Merseyside, which was gutted in a fire in 2002.
Under records which the RMT is obliged to disclose, a further 862 members were listed as working at "unknown" locations, of whom nine had unknown job titles. Some 70 voters were described as "cleaning staff", although Network Rail does not employ any.
Network Rail's chief executive, John Armitt, said: "We believe we have a very good case, as the information provided by the RMT has clearly been inaccurate and deficient."
Union insiders said it was difficult to keep track of railway staff, many of whom have been transferred through a string of different companies since the industry was fragmented under privatisation.
One insider said that, since the reform of laws governing strike ballots, "there's barely been a major or a minor in dustrial action which hasn't had the guv'nor saying 'we'll have to see the lawyer about this, it's dodgy'."
But a rival union, the Transport Salaried Staff Association, took the precaution of writing to all its members to check that their details were up to date before holding a ballot of its members at Network Rail, which is due to be counted today.
