elyob wrote:

Wouldn't it be easy enough to let the gates know which train has come
in and then check the validity? i.e. if it's a fast from Surbiton, no
way could a Vauxhall be valid.

Too many trains come in at once and also, IIRC, passengers are able to move
between platforms behind barriers so there's no way one can fairly assume
what stations a passenger could and couldn't have used. You also have the
problem of the trains not running to time or making unscheduled pick-ups,
let alone interchanges and extensions (e.g. if I come in on a Woking fast
and use my travelcard how does it know I've also got a paper extension?) so
there'd be no end of legitimate passengers held up.