A total of 165 trains are to be removed from schedules every day until the end of July amid "disruption and inconvenience", operator Northern has announced.
The union warned that its members are facing increasing levels of abuse from passengers because of delays and cancellations.
Services in areas including Manchester, Liverpool, Blackpool and the Lake District will be affected by the reductions, which the operator says will improve reliability.
This comes after Sheffield City Region Mayor Dan Jarvis laid into train operator Northern and said using the service was like "going back in time".
I recognise that changes to timetables are likely to cause a small amount of disruption as customers get to grips with new train times, but what we have seen in recent weeks is nothing short of a debacle.
"It has been hard for many of our customers and I am truly sorry for this".
"We are committed to working with Network Rail to get things back on track as quickly as possible, and to deliver the services our customers expect and deserve".
David Sidebottom, passenger director at Transport Focus, said passengers want "accurate information" and called for "an honest, realistic interim plan that leads to a return of reliable services". These changes were billed as a serious increase and improvement in services, but instead it has resulted in chaos, impacting terribly on passengers, both in and outside London and damaging our worldwide reputation as a global city.
RMT general secretary Mick Cash said: "The daily chaos on Britain's railways is down to a toxic combination of corporate incompetence and political failure that can be traced directly to the door of the Transport Secretary Chris Grayling". Passengers have had enough of excuses and a lack of accountability.
Network Rail, GTR and Northern apologised to passengers, blaming the "sheer number of changes" and late-running engineering projects for a delay in approving the new timetables and making amendments.
In a joint statement on Thursday, Network Rail, GTR and Northern said the disruption was due to the biggest modernisation of the railways since the Victorian era.
Mark Carne, Network Rail's chief executive, said there was no doubt the May timetable was finalised "significantly later than normal for reasons that were both within and without our control".
'While this is now a huge inconvenience to passengers as the changes bed in, we are investing in the biggest modernisation of the railway since Victorian times and this new timetable will deliver hundreds more services up and down the country'.
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