I entered Euston Square station at a quiet time and noticed two
gentlemen, in plain clothes, facing the gates from about 3 metres.
I stopped to watch.
Some customers were asked to show their photocards.
Each gate was set to show a light, when used with an Oyster photocard.
I was impressed.
I have not seen any reference to such technology.
"We can tell you use an Oyster photocard." might be a good poster.
Discouraging fare evasion seems better than catching miscreants.
--
Walter Briscoe