The National Union of Rail, Maritime and Transport Workers (RMT) led by Bob Crow announced a 24-hour strike next Friday on CrossCountry Trains, which connect Glasgow, Edinburgh and Aberdeen to England via the East Coast Main Line.
The dispute marks the fourth separate campaign being fought by the union and comes as fresh talks are due to be held today to avert two 24-hour strikes by ScotRail staff on December 22 and Christmas Eve.
In the Orkney Isles, ferry services connecting Orkney to Scrabster across the Pentland Firth were cancelled yesterday as RMT members staged the first of three 24-hour walkouts over a dispute with operating company, Serco NorthLink Ferries.
The Aberdeen-to-Shetland ferry service was also cancelled yesterday, though this was due to poor weather, the company said, with only eight of the 26 crew who were scheduled to work being absent because of strike action.
Signallers working in Stirling for Network Rail have also been on strike since midnight in the latest of a series of 24-hour actions held as part of a long-running dispute over shift patterns.
The union has demanded that staff be allowed to work a three-day week instead of five, with longer, 12-hour shifts.
Network Rail has rejected this, saying it would drive up costs, while insisting the action is not expected to affect services.
The latest dispute on CrossCountry has been triggered by a "comprehensive breakdown in industrial relations", the RMT said.
It said there were six issues at stake, including harassment of staff, unfair sales targets, lack of time for staff training and disciplining train managers for non-attendance of duty on a Sunday.
Mr Crow, the union's high-profile general secretary, rejected accusations by the Institute of Directors that it had co-ordinated the strikes to cause maximum disruption to passengers.
"The idea that RMT has coordinated the various disputes across Scotland is a total lie," he said.
"It is simply an unfortunate coincidence that a series of disputes with aggressive and greedy private companies have come together at the same time and that is more a reflection of the attacks that working people find themselves under this winter."
However, the union has threatened to escalate the row with Network Rail into a national dispute affecting all signalling staff across Scotland unless its demands are met.
The RMT said staff had been subjected to "gross intimidation" by management.
Mark Wallace, head of media relations for the Institute of Directors, claimed the RMT's actions showed it was stuck in the past.
He said: "Many trade unions have adapted to the 21st century and carry out productive negotiations, but it seems the RMT is still stuck in the past. These strikes seem co-ordinated to cause maximum misery to passengers at a time when everyone wants to get their Christmas shopping done and then spend some time with their families."
CrossCountry said it was confident of finding a resolution to the dispute ahead of next Friday's planned strike.
A spokesman added: "Their announcement is regrettable as discussions to resolve the issues raised by the union are ongoing and a meeting is already planned for Tuesday, December 18, where we are confident of a resolution."
ScotRail said it had responded to a request yesterday for further talks regarding the dismissal of ticket collector Scott Lewis. His sacking for gross misconduct triggered a strike announcement on Thursday by the RMT.
The spokesman declined to comment further on talks, which are being held today.
Regarding the Orkney dispute, Stuart Garrett, managing director of Serco NorthLink, said the RMT had "failed to respond" to requests to resolve the stand-off, being held over the introduction of flexible working patterns that will allow the company to cut the level of crew during the quieter winter months.
However, the RMT said staff had been subjected to "gross intimidation" by management.
A further strike on the Shetland-to-Orkney ferry is due tomorrow, with a repeated action planned for all Serco NorthLink services over the next two weekends.