I was racking my brains to recall how the job was done, so thanks to @Dr Hoo for his response ! I do remember that the work was indeed done in stages, leading to some 'interesting' train service patterns; For a while, when the main lines through Newton were blocked, a through service operated between Glasgow Central High Level and Dalmuir, via Kirkhill, Hamilton, Motherwell and the Whifflet/Sunnyside (normally) non-passenger route. No Drivers had route knowledge throughout so there were multiple changes, which in some cases became ridiculous; I recall some trains having relief at Newton, Motherwell, Shettleston and Hyndland. If just one of those Drivers was out of place the service collapsed, and I remember Saturdays being especially bad; It was a Controller's nightmare !