First ScotRail said it was Scotland's biggest electric train order in 10 years
A £180m fleet of electric trains will be introduced on the Scottish rail network.
Siemens will supply 130 new carriages, providing 9,000 more seats, which will be brought into use by the end of 2010.
Finance Secretary John Swinney announced the deal, which will see 134 jobs created to operate and maintain the fleet and the expanded depots.
The trains will operate mainly in the west of Scotland and on the forthcoming Glasgow Airport rail link.
"If we want to encourage more Scots to switch from their cars to public transport, we need to continue to invest in our rail network," Mr Swinney said.
First ScotRail said the new trains would lead to rolling stock being released to operate on the forthcoming Airdrie to Bathgate line, meaning more capacity on services connecting stations between Edinburgh and Glasgow.
HSBC will act as a third party in the deal with Siemens Transportation Systems, which will see the agency Transport Scotland provide £180m for the new fleet.
First ScotRail will then lease them out on a 17-year operating contract.
First ScotRail managing director Mary Grant, said: "This is the biggest electric trains order in Scotland for a decade.
"It will result in extra trains, and services, at a time when more and more people are choosing rail as their preferred means of travel."
Transport Scotland will also provide £40m for a platform extension programme across Ayrshire and Inverclyde to accommodate the longer electric trains.