XC services are very popular at Motherwell. Not only are they faster than the irregular ScotRail services to Edinburgh, but to both Glasgow and Edinburgh their fares are lower (if walk on XC only tickets are used). Motherwell only a couple of years ago was noted by a survey (from one of the major banks) to be the fastest growing Edinburgh commuter town, so removing its links probably wouldn’t be wise, and replacing them with ScotRail services (assuming XC continued running via Shotts) would only serve to place even more capacity problems on Haymarket to Waverley.
Also, I’ve got the new Shotts layout open in front of me (at least the end controlled by WSSC), and at no point do I see a 100mph section. It varies from 60-80mph for most of its length. Add to that it’s very hilly nature which would slow a Diesel Voyager, and any time savings would be minimal.
Slotting any express service into that line would be a nightmare. The current limited stop is often right on the tail of the stopper by the time it gets to Uddingston as there is nowhere to loop a delayed service. As a resident of the eastern end of Motherwell, I’d much rather see the current limited stop service replaced with one that calls at all the busier stations (essentially all the stops, excepting maybe Hartwood, Addiewell and Briech).
XC services should see some improvement from the planned realignment at Carstairs which will remove the horrendous 15mph restriction there.