First electric trains to run on Edinburgh-Glasgow main line on Sunday - a year late
However, they have had to be borrowed from other routes because a brand new fleet for the line will be six months late.
The new Class 385 Hitachi trains are not expected to enter service until late March, as The Scotsman revealed last month.
In the meantime, two Class 380 trains will be drafted in to run some services on ScotRail's flagship route via Falkirk High.
They will be seven coaches long, providing more seating than the three or six-carriage diesel trains they will replace.
ScotRail said the trains would cut journey times because of the electric train's superior acceleration.
It said they would operate more than one in four services on the line.
It said the trains were spares for those which currently operate on routes such as Edinburgh-North Berwick and Dunbar, and Glasgow to Ayr and Gourock.
They should have started running on the main Edinburgh-Glasgow line last December, but the electrification project has been delayed several times.
Problems have included with faulty overhead wire equipment and production difficulties at Hitachi's factory in County Durham.
ScotRail Alliance managing director Alex Hynes has yet to commit to a date for a full electric service on the line, which is due to cut journey times by ten minutes to 42 minutes in December next year.
He told MSPs last month: "I'm not yet prepared to make a commitment [over introduction] because of the complexity of what we are trying to achieve."
Mr Hynes said today: “The running of these electric trains from our existing fleet is an important milestone ahead of the arrival of the brand-new Hitachi trains."
Transport minister Humza Yousaf, who has repeated voiced his frustration at the delays in the past, said: “I’d like to offer my personal thanks to all the many people who have worked on this significant electrification project as their efforts come to fruition.”