David Cantrell wrote:

If you want to seriously reduce fraud, use ticket inspectors. I have
*never* seen one on a tube, almost never on suburban trains, and only
once on a bendy bus.

I've seen them a few times at Liverpool Street (tube) but they invariably
hang around by the barriers and ask to check people's tickets. Far better to
deploy them at ungated stations. As for buses, there have been times when a
cohort wait at a bus stop and pounce on it - it happened on the 25 late one
night the other week. But regularly on board inspectors would work much
better.