TfL's Conditions of Carriage state you must wear a face covering:
- "when travelling on our services" and
- "when in our bus and rail stations, on our platforms"
As Wimbledon, Richmond, Twyford and Slough are not TfL managed stations, I don't think they can classify any part of those stations as "our" stations or "our" platforms. Hence the mandate can't even apply until you board the service.
West Ealing is a TfL Rail managed station, so if you are boarding a TfL Rail service and are travelling under the TfL CoC, you would need to wear a face covering once you pass the entrance to the station building. However, if you are using a paper ticket on your journey, I would say that is a journey made purely under the NRCoT, so TfL's "mandate" doesn't apply to you.
If you are travelling on a GWR service then the TfL CoC are of no relevance to your journey and so again, you don't have to wear a face covering. Of course, you might well intend to travel on a GWR service when you enter the station, and thus not need to wear a face covering, but then decide to take a TfL Rail service that turns up sooner...
It's almost completely unenforceable and TfL know it - Commissioner Andy Byford more or less went so far as to admit this before they decided to keep the mandate.