There are some interesting examples here.
However I'd focus more on regular links to places within an hour's journey (rather than the fact that some smaller places happen to get a daily stop on the Aberdeen - Penzance service, since that one-a-day journey to somewhere far far away is going to be of limited use).
Sometimes it's easy to fall into the trap of comparing apples with oranges to justify population sizes (some places like Bradford and Sheffield have "populations" that include far off villages, some cities like Nottingham and Manchester have tightly drawn boundaries). There are also places that may have a reasonable population but their coastal location means that they don't have much of a hinterland.
I'd suggest Rotherham (which admittedly has an inflated population due to the boundaries going all the way from Brampton to Thorpe Salvin - from the edge of Hoyland to the edge of Shireoaks) - given that there are regular Cross Country and TransPennine services running through the old Masborough station site as well as a handful of EMR London services - but Rotherham's services are...
- 3x Sheffield (split roughly with 10/20/30 gaps rather than a more balanced twenty minute service - I've waited half an hour at Rotherham Central in the past - in the pre-TramTrain era - it's very frustrating!)
- 2x Doncaster (one of which extends to Adwick)
- 1x Leeds (nine intermediate stops, so it takes exactly one hour to do a distance of twenty six miles as the crow flies, thirty two miles as the railway was built)
- Plus a handful of Sheffield - Pontefract - York services a day (and the TramTrain services to Sheffield if you want to include light rail?)
...so nothing south of Sheffield, no services to longer distance destinations like Manchester/ Birmingham/ London (though such trains pass the old Masborough site, a few hundred metres from Rotherham Central, which must make things frustrating for locals!). Generally everything was operated by Sprinters so no First Class or buffets or anything fancy.
In terms of longer distance links, a left field suggestion would be Leicester - I say "left field" because it looks like it has a good service at first glance - it's a junction, it's in the middle of the country, it isn't surrounded by a national park (e.g. Sheffield is a reasonable size but the empty swathe of Peak District on our doorsteps act a bit like the North Sea at Sunderland - a large unpopulated area close to the city)... Leicester is on a busy main line, but doesn't have anything west of Birmingham or north of Sheffield - only two Turbostars per hour to Birmingham seems pretty poor when you consider that they are two large cities thirty five miles apart, so closer than Edinburgh to Glasgow)
Bradford. An express railway backwater with both its stations on dead ends. I have always thought it gets the worst service for a city of its size in England
Bradford is a funny one.
It's generally got six trains per hour to Leeds, two trains per hour to Manchester, hourly services to York /Hull/ Huddersfield/ Chester and Blackpool via Burnley/ Preston, as well as a few London services (the LNER one was going up to bi-hourly, the GC one was a few a day). Plus frequent local links to Skipton/ Keighley/ Ilkley etc.
So even if there was a central station, I don't know what significant improvements it'd get. If anything the Huddersfield service is the one I'd throw an extra train per hour at (locals may argue though).
I can appreciate that people in Bradford look at Leeds getting a significantly better service (in the way that people in Sunderland are envious of Newcastle, I'm sure people in Rotherham think similarly about Sheffield, the grass is always greener), but I think Bradford has a good collection of services and I don't think that linking Forster Square to the Interchange would make much of a difference to Bradford itself.
