run hourly between Huddersfield and Wakefield Kirkgate as this would only require the 1 unit
I could be wrong (often am!), but I thought that one reason for the extension to Castleford was that one unit wasn't able to cope with an hourly Huddersfield - Kirkgate shuttle (given the turnaround times, the paths not matching up perfectly, the slow alignments) - so they decided that if they needed to use a second unit then they might as well extend it through to Castleford rather than sitting clogging up a platform at Wakefield?
As ever, it's frustrating that a 2-car unit that's 20-30% full in the North is ripe for the axe, but an 8-car unit with the same load factor in Surrey can just carry on regardless without even a suggestion of cutting it back to 4-car
An eight coach EMU that's 20-30% full may still be cheaper to operate than a two coach DMU that's 20-30% full, given that the "Surrey" service may be operated on a DOO basis, and the EMUs are more efficient than DMUs (so not only lower fuel costs but also better track access charges)
However, the hypothetical EMU in your example will presumably be full and standing at least a couple of times a day, when providing a service into/ out of central London at peak times (and therefore needs to be eight coaches long at that time - and dumping four coaches somewhere else for part of the day would create shunting moves that are more complicated than just keeping it eight coaches long all day) - whereas a DMU shuttling between Huddersfield and Wakefield isn't going to have the same "peak"
I've lived in Yorkshire for most of my life but I don't think that complaining about Surrey helps the case for the fact that DMUs between Huddersfield and Wakefield have been fairly lightly loaded since I can remember
How about an all stopper York to Manchester via Castleford and Huddersfield?
What would that solve though?
There's (assuming we go back to pre-Covid timetables etc) four per hour from York - Huddersfield - Manchester - a stopper via Castleford is going be overtaken by at least a couple of these - and already an hourly service from York - Wakefield
If there's a way to link them reliably, I'd probably still keep them as separate services publicly, because the old Grand Tour used to confuse people and there was very little through traffic either side of Huddersfield other than enthusiasts.
I agree about the passenger numbers - but I think that the reason that the Grand Tour confused people was that it was (at least sometimes) advertised as a Selby - Wakefield service, so people at Leeds assumed it was a reasonable way to get to Huddersfield/ Wakefield.
Now that the Bradford - Huddersfield service is separate to the Leeds bits, I think that'd be less of an issue