I've been wondering this for years! Sometimes when I show my ticket on the train they clip/mark it in some way and sometimes they don't. There seems to be no pattern to whether they do or not! I've even had Travelcards clipped back in the early '90s (and this was on purely London TC on a journey within London). Are they following some kind of system, or you they just do it when they happen to feel like? :-?