The reason for the split is the difficulty in ensuring reliability over the entire route. Holdups around Liverpool, Castlefield, the Hope Valley (well illustrated this morning), and at Sheffield to the west, then ECML and over the flatter lands towards Norwich in the east.
In an East Midlands presentation a couple of years ago it was stated that 25% of all their delays on all their routes were on this one. One hourly service out of all the more frequent St Pancras and other regional routes.
The Dore bottleneck has been down for fixing since before 2005 and won't be until 2023. Castlegate some time, ever?
It must be a nightmare to manage. When.operated by 158s its quite a pleasant journey. The current worn out 153s and 156s give the service a feel of impending death at present. Users of other Hope Valley services weren't happy to be kept waiting for three quarters of an hour today - neither passengers nor freight!
If it could be made much more punctual, and always operated by clean and reliable modern trains, passenger numbers all the way along the route would improve.