Michael R N Dolbear wrote:
The ability to add the Railcard and thus the railcard capping discount
to your Oyster has been around since 2008 and I got my sister to try it
early that year. She got it done at King's Cross with no problem. Since
she needs to carry the Railcard anyway for her trips to London there
was no additional complication though now she has a English bus pass
she doesn't use her Oyster much in London.

FWIW I looked at this in 2009ish, but I first would have had to register my
Oyster card before I could then add my 16-25 Railcard on top. Since I don't
live in London, and mostly use PAYG without reaching my daily cap, I decided
there wasn't much to be gained.

In addition, paper travelcards (eg those bought with my train ticket to
London) avoid all the issues with touching in/out not recording station
interchanges correctly (and charging penalties) that have been much
discussed here in the recent past.

Theo