The Class 365 electric trains have been drafted in to ease overcrowding after a brand new ScotRail fleet was delayed.
Other trains have gone after their leases expired.
The first services are due to run between Glasgow Queen Street and Edinburgh Waverley via Falkirk High late on Saturday evening.
They will operate the 2230 Glasgow-Edinburgh and 2330 Edinburgh-Glasgow services.
The 23-year-old trains' nickname is because they appear to smile.
They previously ran London commuter services.
BACKGROUND: Eight-carriage "Happy Trains" to ease crowding on Edinburgh-Glasgow lineA memo sent to ScotRail staff from Graham Heald, its head of customer experience, said: "I’m pleased to be able to confirm that our temporary class 365 trains will begin to enter service from late evening on Saturday, 23 June.
"One eight-carriage train (2 x 4-carriage units) will enter service at first, with more trains due to enter service over the coming weeks.
"Once all the trains are in service, seven of the eight trains per hour between Edinburgh and Glasgow via Falkirk High will be electric.
"The entry into service of these trains is the result of a lot of hard work by teams across the ScotRail Alliance, and the effort will not go unnoticed by our customers.
"Once all trains are in service, there will be an extra 17,200 seats per day between our two biggest cities."
However, the trains don't have wheelchair ramps or wi-fi, unlike the diesel trains on the route they will replace.
Station ramps will be used instead.
The trains will also not call at Bishopbriggs or Lenzie because they are too long for the platforms.
Brand-new Class 385 electric trains have been delayed because of problems drivers have encountered seeing signals through their windscreens, which are being replaced.
The first of these trains is expected to carry passengers on the line next month.
They will introduced first at weekends, then off peak before running commuter services.