I would assume that because the concession card is scanned and validated they are simply able to charge back in the same way as any bus operator. For trains. I would again expect that some kind of flat rate and your payment is agreed with operators for the carriage of visually impaired passengers and that now scottrail is effectively nationalised that this only applies to Avanti West coast, London, northeastern Railway and cross country. Certainly my experience on scottrail services since moving south 2 years ago has been that at both stations and on trains. They haven't even been interested in the ticket that I have bought to pay for my journey and simply pass me by. I am not brave enough to test this by not purchasing and then potentially being prosecuted