Train services across large parts of the north of England are set to undergo a major change this weekend.
Rail routes covered by Northern will see current train times and stopping patterns change from Sunday.
Arriva Rail North Ltd, which operates the Northern franchise, says 90% of the services will be affected and there may be some disruption.
The firm operates services across the north from Liverpool, Manchester, Leeds, Sheffield and Newcastle.
Northern said it was "a significant operational challenge" and the biggest change in many years.
It said passengers should check their travel arrangements before embarking on a journey.
The company said there would be an extra 1,300 services added to the network.
Northern's train planners said their efforts were hampered by an electrification delay on parts of the routes "leaving less than four months to fully re-plan our May 2018 timetable, less than half the normal time required".
A spokesman said: "This is a significant operational challenge, and given the late nature of the planning for this, we do expect some localised service disruption, which could happen at very short notice while the new timetable beds in.
"We will continue to do everything we can to ensure we minimise any service disruption and keep customers informed.
"Over the last four weeks we have been focused on ensuring our customers know that timetables are changing with a 'check before you travel' message across stations, trains and online channels."
In April, a regional director of Arriva Rail North firm sent scathing tweets to people who complained about the services.