Only very limited ScotRail services are likely to run on bank holiday Monday, and Tuesday, with all trains cancelled on some lines.
The new ScotRail-Network Rail alliance is expected to announce an emergency timetable today to give passengers as much notice as possible of the threatened disruption.
Members of the Rail, Maritime and Transport union (RMT) and Transport Salaried Staffs Association (TSSA) at Network Rail are due to walk out for 24 hours from 5pm on Monday.
However, services will also have to be cut back for the whole of Monday because the unions will impose an overtime ban which will reduce the number of key staff working.
This could mean no trains running in the northern half of Scotland because lines there have numerous signal boxes.
Network Rail could operate more services around Edinburgh and Glasgow because signalling in the Central Belt is largely handled by two large control centres which could more easily be covered by managers.
A total of 25,000 Network Rail staff are affected, with the RMT representing signallers and track workers – crucial to trains running – and the TSSA representing office staff.
The strike would be the first Britain-wide rail stoppage for 20 years, since a dispute between train drivers’ union Aslef and British Rail in 1995.
A second day of talks between the two sides in the current row was held yesterday at the conciliation service Acas.
Meanwhile, Network Rail has threatened legal action to prevent TSSA members taking part in the action because of “defects” in its ballot.
The union voted by a slim majority for walkouts, compared to the decisive vote by the RMT.
A Network Rail spokesman said: “We have asked the TSSA to withdraw notice of their industrial action as we believe there are numerous defects in their ballot information.
“Network Rail has a responsibility to passengers, freight users and to the country as a whole to do everything we can to avert a strike.
“Talks with the unions continue and we hope to reach a settlement, but we must explore all avenues at our disposal and that includes legal ones.”
TSSA general secretary Manuel Cortes said: “We are frankly puzzled as to why Network Rail are taking this legal route whilst talks continue today at Acas to solve this dispute and avert next week’s strike action.
“They seem more determined to impress [UK Transport Secretary] Patrick McLoughlin and the Tories than making a serious attempt to resolve this pay row and ensure that the trains run on time over the bank holiday.
“We want to settle this around the negotiating table, not in the High Court.”