I have tried my plan today and everything went fine. First I went to Staine Rail Station to colloet my Feltham + London 1-6 off peak travel card, then walked to the nearest bus stop which was about 100 meters away. Got on the bus at 9:41, showed the travel card to the driver and there was no problem. 30 minutes after I arrived at Feltham station bus stop and the railway station was 150 metres crossing the rail tracks. Waited only 5 minute and caught a 10:11(about) train to London Waterloo via Richmond. Coming back at about 4pm, I did the same and the traffic was fine. Tonight I still need to ride the same bus twice so this travel card is really good value to me.
I remember that in some railway stations outside London zones, the railcard ticket will be collected while exiting the station. Whereas this didn't happen in Staines or Feltham as far as I know. That means I can still use the travelcard at the rest of the time in a day.
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Much better IMO as that bus goes along dual carriageways for a significant stretch of the journey. The 117 was a tedious residential route from what I can remember.
Bus 290 takes 45 minutes from Staines to Twickenham whereas bus 117 takes 28 minutes to Feltham. I'd do it to Feltham as train is more comfortable than bus.