New timetables now online at https://be803fe5c416e39d38ae-aa21086260d3bd4e072d597fe09c2e80.ssl.cf3.rackcdn.com/images/timetables/2018-11/december-may/Northern-25-web-1018-28pp.pdf and https://www.tpexpress.co.uk/travel-updates/timetables
The main changes are that the Manchester Piccadilly to Leeds stopping service (the one that calls at Greenfield and Marsden) is to be split into a Manchester Piccadilly to Huddersfield service (which will call at Greenfield, Marsden and also Slaithwaite), and a separate Huddersfield to Leeds stopping service.
In practical terms this means that times at Slaithwaite will be on the opposite half-hour from now. Apart from one early morning service and two evening trains (at Slaithwaite), there will no longer be through trains between Marsden/Slaithwaite and Leeds.
There are also changes to the peak hour trains westbound operated by Northern.
It seems a bit pointless to compare it with the timetable from May to December 2018, as the current timetable bears little relation to reality.
The reasoning behind this is that it will improve reliability, and it’s difficult to argue against this as an objective. The disruption to the peak services to and from Manchester, and to the daytime services, is something which needed to be addressed.
If your daily commute is to and from Leeds, with minor delays but relatively few cancellations, then you might not be aware of just how difficult the Manchester commute has become, with the peak frequency reduced from half-hourly to hourly and with even the reduced service being regularly cancelled or terminated at Stalybridge. The daytime service, too, has been subject to a large number of cancellations, often leaving two, three or even occasionally four hour gaps with no service.
The disappointment is that the one good thing in the May 2018 timetable, the through trains to Leeds, has had to be removed in an attempt to provide the reliability which should never have been lost in the first place.