Network Rail is set to make a High Court bid to stop a crippling strike by signal workers after Easter.
The company has served papers on the Rail, Maritime and Transport union calling into question the validity of its strike ballot and both sides will argue the case before a judge.
Four days of strikes are due to start from next Tuesday in a row over job cuts and work practices, threatening major disruption for tens of thousands of travellers during the Easter holidays.
The legal action comes amid conciliation talks between the RMT and the unions, which are scheduled to continue at Acas.
Announcing its court bid, Network Rail (NR) said: "(We have) a responsibility to all our passengers and freight users, and to the country as a whole, to do everything we can to avert a strike. Talks continue and our aim is a negotiated settlement, but we must explore all avenues at our disposal and that includes legal ones.
"We can confirm that papers have been served on the RMT. This calls into question the validity of its ballot amongst our signallers, highlighting scores of discrepancies and inaccuracies."
NR sources said talks at Acas on the maintenance dispute had been "constructive" and the company hoped they would continue.
The RMT's 12,000 Network Rail maintenance members voted by 77% in favour of strikes earlier this month, while its 6,000 signallers backed industrial action by 54%.
Bob Crow, RMT general secretary, said the union was putting together an "experienced legal team, including some of the most high-profile employment law experts in the country" to fight NR's move and added: "This is a scandalous attempt by Network Rail to use the full weight of the anti-union laws to deny our members their basic human right to withdraw their labour and we will fight this assault on our union to the hilt."
Rail passengers have been warned of "severe" disruption if the action goes ahead, with some rail firms telling their customers that "significantly reduced" services will run from April 6 to 9 if the walkout by thousands of maintenance workers, signallers and supervisors is not averted.