On Sun, 08 Nov 2009 12:33:59 GMT, Neil Williams wrote:

The main thing that makes it difficult is deciding what to deduct on entry.
obviously a First Class 'National pre pay card' would need to hold a balance
of something over £1000 on journey commencement...

You probably wouldn't need to go that far, because mainline services
would presumably retain on-board ticket inspections, where a
non-validated card would presumably receive a penalty, payable
separately.

(By the same rules, you should charge a £150-ish deposit to anyone
buying a paper ticket at Euston to South Hampstead just in case they
go to Manchester).

Not really. The issue with PAYG smartcards would be that you could
touch in at Euston, get checked on a train to Manchester (and be found
to have a validated card), then avoid touching out at Manchester (e.g.
use a different ticket at the barriers, or an ungated station, etc).
What fare should you get charged then?

LU deals with this by charging you the maximum possible fare if you
touch in without touching out. Hence talk of a £1000 charge for
mainline services.