Hundreds of rail workers have walked out on strike in a dispute over the absence of conductors on a new Scottish service.
The RMT union said there was solid support for the action with all of its members in Scotland taking part and picket lines at major stations throughout the country.
Workers are furious at ScotRail's plan to employ ticket inspectors rather than conductors as the second person on the Airdrie-to-Bathgate line, due to open in December.
RMT argues that conductors or guards are best equipped to deal with serious incidents and it warned that the plan would "jeopardise passenger safety."
Regional organiser for Scotland and Northern Ireland Ian Macintyre said members were acting in the interest of passengers.
"They are very determined - they have made it quite clear regarding the safety aspect and they are taking this industrial action in the interest of the public," he said.
"Our members' resolve is quite clear."
RMT general secretary Bob Crow said that the company had refused to give assurances regarding keeping guards on the Airdrie-Bathgate line and on all those lines to be electrified under the Edinburgh Glasgow Improvement Project.
"The company is now blaming Transport Scotland for this situation and relying on the fact that it will be indemnified against any strike action by the taxpayer," he said.
"First ScotRail and the Scottish government are still refusing to face up to their responsibilities to the travelling public and their staff and are prepared to slash corners on rail safety in the dash for cuts and the drive for profits."
ScotRail claimed it was running at least 95 per cent of services and that it was business as usual for Scotland's railway.
Managing director Steve Montgomery said: "We are running even more services than during the last strike.
"At least 95 per cent of services are operating, with the expansion including all Glasgow-Oban trains. It is business as usual for Scotland's railway."
The strikes will be spread over four days. RMT guards and drivers will strike to the end of Wednesday.
Sleeper-train managers will stage a two-day walkout from 6pm on Tuesday to the same time on Thursday.
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