The threat of the first national rail strike for 14 years was raised when the industry's biggest union announced that it was balloting 17,000 workers for industrial action.
The action would cripple train services across Britain.
The Rail Maritime and Transport Union (RMT) said maintenance and signalling staff will vote over the next week on whether to launch a campaign of industrial action in two separate disputes.
The result of both ballots will be known on May 22 and strikes could start a week later, right at the beginning of the holiday season.
The union warned that if the strikes go ahead the railway system would be paralysed.
Ballot papers will be sent to more than 12,000 infrastructure workers after they rejected an "unacceptable" offer from Network Rail on harmonising terms and conditions.
In another row, 5,000 signal workers and other operational staff will be asked if they want to strike over pay and conditions after turning down an improved offer the union said was worth just 0.1% in the first year of a two-year pay deal.
The harmonisation dispute follows months of talks aimed at achieving a single set of terms and conditions for maintenance staff, many of whom have transferred to Network Rail from private firms.
Bob Crow, general secretary of the RMT, said: "The company has been using the talks to drive down our members' conditions and they can hardly be surprised that their pathetic offer was thrown out by a margin of more than 100 to one.
"The company is now saying that our members can stay on their existing terms but they are already moving to sneak inferior conditions in through the back door. We know that means an attack on everyone's terms and conditions, not least because the company is looking to cut its maintenance budget by up to 12% year on year."
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