As the OP of this thread, can I clarify that I don't have an issue with showing my railcard, my point was just that in the last nine years since I started using trains a lot (probably average a dozen times a month), I have NEVER been asked to show it at a gateline before. Coupled with the insistence that he wanted to see our 16-25 railcards (and no, there couldn't be any confusion!), it struck me as quite a strange request.
A lot of these replies relate to showing a railcard on the train, whether asked for it or not, which is a different situation altogether, and of course most people would not have any problem with this.