Passengers on the main Glasgow to Edinburgh rail route through Falkirk face crowded trains from a week on Monday (February 26).
The late arrival of a new fleet to service the line, ScotRail’s busiest, will leave the operator short because the lease on four of its current trains expires in weeks.
ScotRail is so concerned at potential overcrowding that it will slash fares on another route between the cities by nearly half in an attempt to persuade commuters to switch.
The firm said it expected up to 12 trains a day to run with fewer carriages than normal from Monday, February 26 to May 20.
It said those peak hour trains, which normally operate with six carriages, would only have three or four. That is the equivalent of at least fewer 1,500 seats a day, since each of the carriages being lost has 63 seats
ScotRail will try to offset the impact by dramatically cutting fares on a secondary Edinburgh-Glasgow route via Bathgate and Airdrie.
Peak-hour return tickets will be cut by nearly half to £13.
The offer starts on Monday, a week before the carriages start disappearing. However, journeys take around 20 minutes longer than the 50 or so minutes on the main line, and there are no catering trollies or tables.
The problem has been caused by ScotRail’s lease on four three-carriage class 170 diesel trains expiring at the end of the month.
They were due to be replaced by the first of a fleet of 70 new class 385 electric trains, but these have been delayed by a series of problems.
A ScotRail spokesman said: “As we prepare for our brand new Hitachi class 385 trains, we are making some changes to our Edinburgh-Glasgow via Falkirk High service.
“This will result in a small number of services having a reduced number of seats.
“Customers should plan their journey in advance.
“We are sorry for any inconvenience this will cause customers”.