Question for Oyster users (specifically travelcards), since it's not
particularly clear from the website! Here's an example trip:

- Get on Central line at Northolt
- Take Central line to Greenford
- Change to FGW Link to Ealing Broadway
- Take District Line to Ealing Common

Where exactly does Oyster need to be touched? Just at the start and
end of the journeys, or also at the NR Oyster validators by the sides
of their platforms? What about if I get on at a tube-only station, get
off at a National Rail-only station? Where should Oyster be waved
then?