Roland Perry wrote:
Although MML at weekends have been known to announce the availability of
their "Weekend First" upgrades, on the train, and collect the money
while doing the ticket check.

One did that when we went to Chelmsford for V-festival. ISTR it wasn't
much either.

If the train is a service that always has a ticket check (like most
Inter-city services), I'd expect there to be a good chance I could
upgrade as fare evasion is, in theory, impossible.

However, on a suburban commuter route where the ticket checking is
usually at the station, it's obvious it would be abused. Therefore the
only logical solution is to issue a penalty fare, or introduce some
clever, and expensive, 'ticket by SMS' option that allows you to send a
text to upgrade or buy a ticket and get a reference number that can be
shown and confirmed - proving you did so when you boarded and not one
second before the RPI turned up! Seems like a lot of effort though!

Jonathan