One of the lobbying groups did a study perhaps 15 years ago into a National Railcard. Their conclusion was that a railcard available to anyone could be revenue-positive, and the modelling suggested (IIRC) that the most favourable for railway finances was a £50 up-front cost for one year, giving one-third discount on Off-Peak travel.
Infrequent travellers and commuters would still pay full fare, but the expected stimulus to demand for off-peak travel was projected to be worth more than the loss in revenue.
As far as a low-income railcard goes... one might suggest that a fairer approach would be to set fares at a sensible level, and use the tax and benefit systems for redistributive purposes. What we currently have is an agglomeration of bits and pieces that don't work coherently, leading to the various 'income traps' mentioned above.
Ofcourse £50 15 years ago is something like £80 today, and obviously fares have increased faster than inflation.
So to achieve the same impact you are probably pushing £90.
Very different to the current Railcard system
£80 - £90 could still work for a National railcard. Of course this is more aimed at being commercially beneficial, rather than removing barriers (you could of course make the card cheaper for the groups with the biggest barriers)
I feel that they'd have to choose between this, or my income related railcard, rather than have both. The industry would have to get over its squeamishness about some wealthy people getting a good deal, however on the plus side, there would be fewer eligibility checks and less data handling.
Either would be better than the status quo. Actually, I remember seeing the said report, however the privatised railway, for all its supposed dynamism, never managed to do anything with it.
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By removing railcard it would make rail ticketing a little less confusing than it is now.
Realistically passengers prioritise affordability over simplicity - however they don't want a system designed to catch them out.