The cost is not significant compared with the rollout of things like TVMs.
So you never change anything because "we don't do it that way here", basically? I despair.
What, like it (OK, an electronic version of it) "doesn't" work on TfL services and Edinburgh trams?
I'm pointing out why it won't work (and therefore address your question of why it was never considered), not why it can't be done (apologies for the ambiguity). There's an ideal world and there's the real world.
I'm of course open to change, but if I'm honest I'd much rather the railway industry invests its limited resources in properly combating fare evasion by conducting regular ticket checks on board and providing adequate facilities to purchase tickets in the first place, rather than waste their time and money installing needless validators which the majority of passengers are likely to simply ignore because they are not used to them. By way of illustration, you only have to look to many parts of the north to see how bypassing a fully functional TVM and buying on board is considered completely legitimate (and in some cases actively encouraged by TOCs).
It's worked with TfL because, for the most part, tickets are stored in an electronic format on a smartcard and the process of validation is intrinsically linked to a barrier line with automatic penalties applied where journeys are not sufficiently validated at both ends. So perhaps blanket smart ticketing in this fashion would achieve your desired outcome more effectively?
In the mean time (and another example of why 'compostage' a la France won't work in the UK) what's the point in putting validators at every station when TOCs can't be bothered ensure every station has adequate ticketing facilities? How does one validate a ticket before boarding which hasn't been able to purchased yet? We could say they should have ticket facilities at every station (which I'm a supporter of), but it's pretty clear this isn't the will of the railway industry in this country.
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Oh right. Wasn't quite so easy to tell from the picture. Anyway, here is one with pre-printed stock which is similar to what the railways need.
I was impressed with the tickets issued to me when I was in Switzerland last year. Thermal printed, had a hologram running up one side, a watermark and quite a complex background which obviously makes it very difficult to copy or forge. But, despite being regularly told of the cost of fare evasion and ticket fraud to the railway industry in this country, I'm guessing it isn't going to make the necessary investment.