Pay as you go, contactless, rail travel in Cornwall (all stations west of, and including Plymouth) has now arrived-
https://www.gwr.com/your-tickets/smart-tickets/payg/cornwall
The three slight drawbacks are that-
1. You have to create an account with GWR (sometimes easier said than done from overseas) then apply for a smartcard to be mailed out to you. You then travel on the smartcard (which is linked to your bank account). That enables you to register and benefit from National Railcard discounts while travelling contactless.
2. Walk up Advance Single fares are not included in the scheme- which may in some cases be cheaper.
3. The wording is very vague and inconclusive as to whether fares are capped at the price of a Cornwall Day Ranger ticket (£17 or £11.30 with a railcard) so in some cases a Paper Day Ranger may be the cheapest option.