I had heard that they ripped up all that double track years ago and sold it off to railways in the US. Between Jasper and Blue River the valleys are wide so there is room to double track. Some area's between Red Pass and Mt Robson ( Foster and Old Foster sidings) are steep and rugged and require extensive mountain moving to put a second track. Once you get into the Ashcroft Subdivision it's labor intensive to build - especially around White Canyon and the Fraser River ( Jackass Mountain) area.
https://youtu.be/DrSJwoD3dls