Signallers and maintenance workers across the UK were already due to stage four days of action from next Tuesday.
And now ScotRail conductors will walk out for three days, from Monday to Wednesday the following week, overlapping with a strike by sleeper train managers that week.
Members of the RMT union are angry at ScotRail's plan to employ ticket examiners rather than conductors as the second person on the new Airdrie to Bathgate line.
ScotRail said the signallers' strike would force the cancellation of two-thirds of its services, with trains largely confined to the central belt between 7am and 7pm.
But the company said it planned to draft managers in the following week and hoped to keep 95 per cent of trains running.
Network Rail was going to the High Court in London today in a bid to avert next week's national strike. It claims to have uncovered scores of "inaccuracies and deficiencies" in the RMT strike ballot.
Among the alleged inaccuracies included a claim that 11 signal boxes balloted by the RMT that do not exist and that 26 workplaces were completely missed out.
The union said it would strongly defend the legal action.