Neil Williams wrote:

It would need to be made obvious to start with, but that could be done
by having, say, a green reader on the ticket machine for touch-in and
a red one on the left hand side of the doors for touch-out. Indeed,
it'd be simpler and more consistent than the London "remember to
always touch in and out, unless it's a bus in which case only touch
in, and unless it's a bendy bus and you have a season ticket then you
don't need to at all", which is unnecessarily complicated.

You've left out what a 15 year old who lives outside Greater London
needs to do to exit Wimbledon station having arrived by tram ;-)

--
Arthur Figgis Surrey, UK