John B wrote:

After about a year, I've finally worked out what they mean: "we don't
want to have to give all our station bods network-wide a list of
numbers to tick off against people who claim they need to go through
the gate to get on the Megatrain, and we certainly don't want any old
fare-dodger to be able to pretend to have a Megatrain not-ticket as an
excuse".

You give them too much credit. I think the real answer can be found he

http://tutor2u.net/economics/revisio...imination.html

Airlines charging more to go AAA-BBB than to go AAA-BBB-CCC has long
been a classic example of price discrimination.
--
Michael Hoffman