Dare I even ask, what is the current journey time of a diverted express train on the Caldervale line? Could an express train ever achieve Leeds to Victoria in 40 minutes without billions of spending? Probably not I guess but it is possible it would reduce the capacity issue.
I assume you mean the Calder Valley line from Manchester to Thornhill Jn and then the Dewsbury line, not the Calder Valley line Great Way Round via Normanton. The best time possible on the Huddersfield line seems to be 48 minutes including 1½ minutes stopping at Hudderfield and a 2-minute recovery allowance, which suggests to me a non-stop time of 46 minutes over a route that has already seen some measure of improvement.
A night-time train (1P02) runs non-stop through Victoria and then via the Calder Valley and Dewsbury. This passes Victoria at 0106 and reaches Leeds at 0158½, including the same 2-minute recovery allowance as the day-time trains, so 52½ minutes or 53½ minutes if starting from Victoria, which makes 7½ minutes more than via Huddersfield. But who knows what could be achieved over the Calder Valley line? It is one of the earliest main lines, characterised by gentle curvature, which suggests that high speeds ought to be possible. Has anyone seen a curve-diagram for the line?
There are at present two severe speed-restrictions, at Rochdale to 30 down and 40 up, and on the Charlestown Curve from 22¼ to 22⅓ miles to 55. Otherwise, speeds have recently been raised to a pretty jagged profile between Manchester and Rochdale but remain as they have been for very many years north of Rochdale, with 70 to the boundary at 22¾ miles and then 60 to Heaton Lodge Jns at 37¼/37.6 miles. There are two breaks down to 65 in the 70 area, from 16½ to 17, and from 18¾ to 19½. There is also, of course, a distance-penalty for using the Calder Valley route.
It would be fascinating to know if any study has ever been done of just what maximum speeds could be obtained on the Calder Valley line, and what journey-times these would deliver, though it does seem pretty unlikely that 13-odd minutes could be saved to get to 40 minutes. But then getting 8 minutes out of the Huddersfield line timings to get 40 minutes including the Huddersfield stop seems likely to cost a pretty substantial sum.