MIG wrote:

(If people really had to leap out of trains to touch validators at
places like Earls Court it would hardly be healthy or safe. Would
they leave their luggage on the train? It would be too bonkers even
for TfL under the rules of Oyster.)

It happens already with some National Rail services - Stratford to Liverpool
Street is covered by PAYG but the barrier readers at stations further up the
line only read for travelcard validity so anyone trying to get an extension
has to do this leaping. It gets worse if the trains use the fast platforms
at Stratford.