Charles Ellson wrote:

You can use PAYG on a Southern train to Harrow, wait 7-ish minutes
then jump on a following London Midland train to Euston. I suspect
many people will decide that "a train is a train" or won't recognise
the difference anyway, resulting in journeys being attempted all the
way on the main line Euston trains.

I don't tend to venture out to Watford Junction much, but just how are
passengers supposed to tell at a glance which are and aren't the valid
trains? Many TOCs seem to have numerous liveries in use due to stock not
having been repainted or a change of mind mid franchise and so forth. And a
lot of announcements don't give the TOC. So how will the passenger at
Watford Junction who knows they can use PAYG "on trains" or "on overground"
(meaning "national rail") know which train to Harrow & Wealdstone is and
isn't valid?