ScotRail is in a race against time to find more stand-in trains to avert further overcrowding because of delays to a brand new fleet.
Hitachi electric trains which should have started running on the Edinburgh-Glasgow main line last September are not now expected in service for several more months.
However, four of ScotRail’s older trains have already left Scotland because their leases have expired.
Three more are due to go in July or August, followed by others at the end of the year.
The shortage of carriages has left some rush-hour services between Edinburgh and Glasgow with half their normal number of seats.
Commuters have reported overcrowding despite ScotRail nearly halving peak fares on a secondary route to relieve the pressure.
The latest hold-up to the new fleet has been caused by drivers saying they cannot see signals properly through windscreens, as Scotland on Sunday revealed last month.
ScotRail is expecting to receive proposed modifications shortly.
Their feasibility and likely timescale is not known.
A Hitachi spokesman said: “We are currently working with manufacturers and partners on viable solutions.
“Once the solution is agreed, windscreens will undergo rigorous testing, working with ScotRail, regulators and unions, before the trains enter service.”
A ScotRail spokesman said: “We are also looking at other immediate options to increase the number of trains while we wait for the new Hitachi trains.
“Our focus remains on helping Hitachi to safely introduce these brand new electric trains into passenger service as soon as possible.”
A spokeswoman for the Scottish Government’s Transport Scotland agency, which controls the ScotRail franchise, said: “Passengers and ministers are equally frustrated that these new trains have not yet entered service, particularly as the line is electrified and already has some existing longer trains running.
“While officials and ScotRail have worked hard to alter existing leases and secure more rolling stock, it is imperative Hitachi identify a swift solution to the current and well-documented technical problems which are delaying service introduction, as well as other manufacturing issues.”