I'm not sure what problem the OP is trying to solve here - it seems quite a complicated "solution" (building new sections of railway) to find capacity, when there are only two other trains an hour from Leeds to Woodlesford and two other trains an hour from Barnsley to Meadowhall (one of which has also served the Wakefield Kirkgate to Barnsley section) - so it's not like there's not plenty of spare capacity on the route that the semi-fasts do take.
Running through the Rotherham loop (as the OP suggests) would add more trains to a bottleneck (flat crossing at the north end of the loop, single track at Holmes, flat crossing at the south end), on top of the flat crossings at Moorthorpe and Swinton.
For the cost of a new line in the Pontefract area, we could have a few grade separated junctions and a double track line to Rotherham which would be of much more practical use for many more people each hour.
The best & cheapest way to improve capacity is easy - provide longer trains - at least 6 to 8 coaches instead of 4 car Voyagers.
True.
The problem that we have is that the length of the hourly (Glasgow) Edinburgh - Plymouth service means its hard to add extra capacity to certain bits - if it were a shorter service then it'd be easier.
I've suggested in the past that we run the Edinburgh service via Doncaster instead and replace the current Newcastle - Doncaster - Birmingham (etc) service with one that starts at Leeds (freeing up a Newcastle - Leeds path for a TPE EMU), which would allow the Edinburgh service to be faster to Birmingham and would allow the Leeds - Sheffield/ Birmingham service to be longer (maybe three fewer Voyagers required to run that service pattern, with a few more TPE EMUs required -that should be easier to source).
Electrify from Fitzwilliam and Doncaster to Sheffield asap as well so the sheffield stoppers can go over to EMUs as well
Agreed - having the same traction on the Leeds - Fitzwilliam - Sheffield service as the Leeds - Fitzwilliam - Doncaster service would mean a faster service from Leeds to Fitzwilliam, plus a more consistent journey time between the two stoppers (making it easier to path the "fast" services).
It may even get the Leeds - Fitzwilliam - Sheffield service (approx 1h15) down to something fairly close to the "semi fasts" between the cities (around the hour mark), which would smooth passenger flows better.
Whilst there are other things not on the CP5 list, it's frustrating that this (forty mile?) bit isn't currently on the agenda.