The operator also plans to switch more trains to its busiest routes, including Edinburgh-North Berwick, to help reduce the commuter crush.
News of the changes come days after ScotRail announced that two daily rush hour services on the Borders Railway would be doubled in length from next week to help cope with passenger demand.
Abellio, which runs ScotRail, ordered 70 new trains when it won the franchise two years ago, but the first of its two fleets will not start arriving until next autumn, and the second is now delayed until the second half of 2018.
It comes at a time of rail travel becoming increasingly popular, with passenger numbers up by one third over the last decade but hardly any extra trains being added to the network.
A Scottish survey by watchdog Transport Focus has shown that getting a seat is passengers’ second top priority for improvement after ticket prices.
The shortage was laid bare a year ago when Scotland on Sunday revealed ScotRail’s busiest trains were carrying more than twice as many people as there were seats.
It has pledged to provide an update in the New Year.
ScotRail is considering acquiring up to another three three-carriage electric trains as a stop-gap measure until the new fleets arrive.
The Class 321 trains are similar to ones which run on suburban routes across Glasgow. They would come on top of an extra seven Class 320 trains which are being temporarily added to the fleet, the sixth of which entered service last month after refurbishment.
These will enable ScotRail to increase the length of some electric trains on the busy North Berwick line by half from four to six carriages.
However, ScotRail has drawn a blank in its Britain-wide quest for more trains on diesel routes, which are under even greater pressure.
It had hoped to get a fleet of eight from London but they do not have toilets and could not run on all routes.
An alternative plan to acquire five trains from Great Western Railway was also foiled because the operator decided to keep hold of them because of delays with electrification of its main line.
A ScotRail source said: “We are doing everything possible to try and bring in more seats as soon as we can. We’ve got the huge expansion coming from September next year, but people want action now. We’ve been trying to get our hands on diesel trains, but that’s been made pretty difficult due to the delays on electrification down south. We’re also looking to see what we can do to get more carriages down into North Berwick.
“We’ll hopefully be able to do something soon, but we’ve a lot of hoops to jump through in terms of getting the safety regime signed off for the new way of working agreed at the end of the conductors’ dispute.”
Transport Focus passenger manager Robert Samson said: “We welcome ScotRail’s efforts to source other rolling stock and hope it’s successful.
“One of passengers’ top priorities is getting a seat, but it is getting more and more difficult as the number of people travelling continues to grow.”