On Tue, 5 May 2009 16:17:40 -0700 (PDT), D DB 90001 wrote:

Surely they are not expecting passengers to validate oysters in the
middle of the journy?!!!! It's complicated enough at interchange
stations about whether you need to touch in or out, making it
compulsory to validate the oyster in the middle of a journey where
there are no barriers would be even more confusing. Surely if you
touch in at Wimbledon and out at Heathrow the system would be
configured so that a greater journey time was allowed to take account
of the doubling back. Why should it be up to the passenger to take
responsiblity for it when it could be automated. I seriously doubt
that there would be a mandatory oyster validation requirement for
passengers mid-journey. That's non-sensical.

From Paul C's informative post, it appears that the reason for the
software upgrade at stations like Rayners Lane is to allow passengers
to get a cheaper fare by taking a longer route avoiding Zone 1, rather
than anything to do with variable time limits.

This would still involve validation mid-journey, of course.

This still leaves a few unexplained stations on the list (Mile End,
Earl's Court, Bond Street, etc), but perhaps a few extra ones were
just included for testing purposes.