How do you know what route a person took?
The same way as Oyster knows - i.e. one default route, and other alternative routes based on barriers passed through and pink readers tapped. Journey time could also be taken into consideration, e.g. some tap in/out combinations will mean there's only one train or route you could have taken.
In a sense this issue is no different to the enforcement of route or TOC restricted tickets. If the TOCs want to ensure people are paying the right fare and the only way of enforcing it is to do on-board checks (validations in this case), then that will be what is necessary!
How will discounts like railcards be applied?
Apply it through an online account or ticket machine. This really isn't the biggest blocker.
Contactless works on largely stopping services in well-defined zonal areas. It works well in London but even that has its complexities (pink readers anyone?). Scaling that up will be very difficult given our plethora of routes, TOCs and pricing models. From London to Birmingham you have 3 TOCs on 2 routes and whilst I'm sure the system could tell how long a journey took it won't tell you a route. One of my larger concerns is how to handle break of journey. Suppose I buy a ticket from New St to Lichfield TV I'm allowed to disembark anywhere en route as long as I complete the journey on time - how will you go that with a touch in/out system? On top of that it doesn't help that the pricing model pushes people to book in advance rather than turn up and go with significantly cheaper fares.
Don't get me wrong, I like the idea and it certainly can be done but I just don't see the will being there from either passengers or politicians. The Dutch have a zonal system for the country after all
See above - there will obviously be winners and losers with any PAYG system. Just as with Oyster now, if doing a BoJ you may be better off buying a paper ticket.
This is almost completely the opposite of an Advance-only system, because you'd simply load Advances to a smartcard or show them on an e-ticket, and this would be for flexible travel.
I agree that this would of course work easiest in zoned metropolitan areas, but many other European countries have found ways of moving to zonally priced travel. The difference here is that no-one wants to grasp the nettle of change.
To be serious, one of the biggest issues is what happens if you don't touch in or out. In London if you forget to touch in or out you get charged a maximum fare. I can't remember exactly how much it is but it's something like £15. Enough to protect revenue but not so high to be too punitive. Remember that even in London many stations don't have ticket barriers and the system relies on tapping in and out on standalone validators.
Let's scale up contactless across the country. How much should the maximum fare be. Let's say I touch in at Penzance, travel to Wick and forget to touch out you might expect the maximum fare to be something like £250, maybe more. Now say I tap in at Penzance and travel one stop to St Erth but forget to tap out. Charging a maximum fare of £250 would be totally disproportionate. Remember you can't have different levels of maximum fares for forgetting to tap out, one size has to fit all.
SWR Tap2Go has a maximum fare of £25. The most expensive single fare within their network is £82.10. Clearly it's a risk, but one they (and the DfT) were willing to take.
At the end of the day, unless all stations are fully barriered, on-train/station revenue inspection will still be required. Just as they are now.
I can't help feeling that people are trying to think of problems, when in fact there are just as many (if not more) problems with the current system of paper tickets.
People would say you're mad if you told them you were introducing a new system where people could pay whatever fare they wanted to get through the barriers, and it's effectively taken on trust unless you encounter an inspection...
SWR Tap2Go quite clearly demonstrates that non-metro PAYG is possible.