Traveling from, say, Chiswick Park to, say, Uxbridge, you
can opt to travel through zone 1, or avoid it by going
on the Rayner's Lane branch of the Piccadilly line.

Are there (still?) different fares for doing this via/not
Z1 ?

If so, and I use Oyster Pre Pay, all I'll do is wipe the
card in at CP and out again at Uxbridge. How will it
know how I got there, and how much it should charge ?

With a paper ticket, you have to make the decision of
how much to pay before you travel, so if an RPI catches
you on the "expensive" route with a "cheap" ticket, you'll
get PFed.

However, with OPP, the funds aren't deducted until after
the event, so the RPI has no way of knowing whether the traveler
was attempting to defraud or not.

Are there any intermediate PP validators for resolving this sort
of thing, so that I'd have to wipe the OPP card at somewhere between
Acton Town and Rayner's Lane to ensure I get the cheaper fare ?

... or am I imagining the whole thing, and all intra-LUL fares can
be computed on origin+destination alone ?

Richard [in PO7]