The Calder Valley (and Rochdale Canal) route suffered from Beeching in being downgraded in favour of the assessed primary route via Diggle, between Manchester and Leeds. This went too far, as did much outside of the South East. Major population centres can merit multiple routes between them because of intermediate traffic, like Glasgow and Edinburgh (and Liverpool and Manchester).
The L&Y route, described in Nock's "Lancashire and Yorkshire Railway" is a much better route across the Pennines, mostly with gentle gradients of c1:300 (and at one time water troughs!), except for a modest climb to Castleton and descent from Summit Tunnel. It is an early George Stevenson line with gentle curves so should be capable of upgrading to 100/125mph. However it is only two track and must provide for the sizable settlements spaced along the route. Faster trains would need to be able to overtake. Rochdale and Halifax would have helped but they have been cut back savagely in a way that BR and DfT would never have dared, South of Watford.
While I would not suggest this as an alternative to the TP upgrade, surely the aim should be to get the best out of what this line can offer, particularly for Bradford's sake.
The detailed service other posters are advocating would be thus enabled.
WAO