...
A good example of a line where delays quickly multiply is Bristol to Taunton due to the mix of fast and stopping services and single-track Weston Loop and junctions either end. Often it is a late-running westbound Cross-country service to that starts the rot. It passes Uphill Junction late, so a Cardiff - Taunton can't come off the Weston Loop, so the next Taunton - Cardiff is held on the main line because it can't go onto the Weston Loop, and possibly a fast service then gets stuck behind that. And then the delayed Cardiff - Taunton has a short turnaround time and is consequently late coming back.
...
A good example of a line where delays quickly multiply is Bristol to Taunton due to the mix of fast and stopping services and single-track Weston Loop and junctions either end. Often it is a late-running westbound Cross-country service to that starts the rot. It passes Uphill Junction late, so a Cardiff - Taunton can't come off the Weston Loop, so the next Taunton - Cardiff is held on the main line because it can't go onto the Weston Loop, and possibly a fast service then gets stuck behind that. And then the delayed Cardiff - Taunton has a short turnaround time and is consequently late coming back.
...
...and I'm sure we can all think of other examples where singling has led to a lack of resilience. When lines are singled to save money, their capacity is not halved - it is reduced by one or more orders of magnitude.