Like most on here, I think that the S-E Bowling curve isn't the best of ideas. We don't need the Calderdale line as a fast alternative to the transpennine route through Huddersfield.
That said, a bit of four-tracking on the existing alignments between Bradford Interchange and Leeds would open up possibilities for a few new stations on this stretch, namely Wortley, Stanningley and Laisterdyke. The four-tracking would allow a Leeds to Bradford Interchange shuttle serving these (plus Bramley and New Pudsey, obviously) a number of times per hour, with Leeds to Manchester services speeding on by. Maybe the shuttle could extend to Halifax, even if the four-tracking doesn't. In this case, maybe it could justify new/reopened stations at West Bowling, Wyke and Hipperholme.
Some sort of service pattern like this, is what I envisage:
- Leeds, Wortley, Bramley, Stanningley, New Pudsey, Laisterdyke, Bradford Interchange
- Leeds, Bradford Interchange, Bradford West Bowling, Low Moor, Wyke, Hipperholme, Halifax, Sowerby Bridge, Myholmroyd, Hebden Bridge, Todmorden, Rochdale, Manchester Victoria
- Leeds, Bradford Interchange, Halifax, Todmorden, Walsden, Littleborough, Smithy Bridge, Rochdale, Castleton, Mills Hill, Moston, Manchester Victoria
(Apologies if I've missed stations out - it wasn't intentional. Obviously I've excluded services which leave the Calderdale line from the above, eg at Todmorden. I'm not saying whether the above should be hourly, twice-hourly or what.)
Going a bit more radical, I also envisage a new chord at the Armley Gyratory, to remove conflict with the Leeds-Wakefield line, and new approach lines for the Airedale, Wharfedale and Harrogate Lines into Leeds Station. These would also probably require new platforms north of the existing platform 0 at Leeds. These would require fairly minimal land take, especially on land already built on. It would also give the step change in frequency that would be required to really tempt people out of their cars.
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Going even more radical, we could start talking about reopening the line from Low Moor for a Bradford-Wakefield service calling at Low Moor, Cleckheaton, Liversedge, Heckmondwike, Dewsbury Moor, Thornhill Lees, Horbury Bridge, Wakefield Kirkgate and Wakefield Westgate. However, that would be a) outside the scope of this, and b) probably a bit too radical!