The operator announced today it had agreed a lease for ten stop-gap trains for the main Edinburgh-Glasgow line.
The four-coach Class 365 electric trains, which can be coupled together, will be an increase on the current maximum six and seven-coach trains.
The so-called “Happy Trains” - because they appear to smile - have been drafted in to plug gaps caused by delays to ScotRail’s brand new Hitachi fleet and leases expiring on other trains.
This has forced the firm to shorten some peak-hour services.
The trains are expected to operate two of the four hourly services between the cities. A third is already run by other electric trains.
The fourth will continue to be operated by one of the existing diesel trains, which are to be replaced.
The Class 365s are due to carry passengers by the end of the month, with the first Hitachi train expected to follow suit next month.
ScotRail Alliance managing director Alex Hynes said: “I am delighted we have secured an extra ten electric trains to boost the number of seats available to our customers by 17,200 a day.
"This is great news for our customers travelling between Edinburgh and Glasgow.
“Securing these extra trains has been made possible thanks to a lot of our people across the ScotRail Alliance working flat out in recent weeks.
Transport minister Humza Yousaf said: "The 17,200 extra seats each day - that’s around 800 an hour - ensures a capacity boost between Edinburgh-Glasgow to support our major events in Scotland this summer.
“The recent arrival of the first four class 365 trains in Scotland is welcome news, enabling staff training to progress more quickly as well as providing more work at Springburn to prepare the fleet.
“We thank passengers for their patience on those routes particularly impacted by capacity issues and assure them these class 365 trains will help alleviate this issue.”