Well, not a scientific answer, but whenever I travel on the route at weekends -particularly between Reading & Paddington- there's always some engineering work going on somewhere that leads to the Main or the Relief lines being shut between X & Y.
And when you consider all the things that are going on as part of GWML total route modernisation, it's scarcely surprising: electrification including associated bridge reconstruction; Crossrail; Reading rebuilding; re-signalling; new depots for IEP; track relaying. I think FGW perform minor miracles every weekend to run anything at all!
In short, you'll probably find yourself weaving from fast to slow lines and back again somewhere, but there's an awful lot of progress to be seen from the train window.....
