"Graham Harrison" wrote

Reading is closed. What you saw was a divert from the west country.
Normal route to Westbury then reverse and down to Salisbury, Basingstoke,
Addlestone, Brentford to get in to the "correct" side of Waterloo. Going
back they use Richmond instead of Barnes. From Bristol/Swansea they
route to Oxford, Banbury (reverse) then down through High Wycombe to
Paddington

also bridge works where the Brighton Main Line crosses the South Western
Main Line means that capacity on the SWML has been limited, and the FGW
diversions have to come in on the Windsor Lines.

Peter