In message , Robin
writes

These systems all seem to involve no "touch out" at all. So I would
predict a massive increase in fare evasion if introduced here.
Londoners are not Scandinavians.

It's also the case that in some European countries, even accidental fare
evasion often results in a massive on-the-spot fine (typically 50 euros
in Italy for failing to validate a train ticket), which is nearly twice
the amount of a promptly-paid TfL "penalty fare".

--
Paul Terry