W14_Fishbourne writes:

Secondly, trying to work out the correct fare for your PAYG journey is
difficult enough on London Underground, with a fairly simple route and
fares structure, never mind on National Rail with its mass of
different routes and fares. (If you want to know what I mean, take a
journey from Gunnersbury to Hanger Lane via Turnham Green, Ealing
Broadway, and North Acton. You won't have stepped outside zone 3 but
just see what fare you get charged.)

The solution to that is relatively simple. Make it like London Buses and
make people touch in when entering (and maybe leaving) a train rather
than (or as well as) at the barrier line at the stations. That way the
exact journey taken is recorded, you are charged the appropriate fare
and it is allocated to the TOC(s) which you actually used rather than on
a statistical 'divy up' basis. This would probably work best if rather
than the 'money' being stored on the card it worked more like a
credit/debit card with each journey debiting the user's account on a
central system.