Always wondered about this. Seems to be different depending who you ask. For instance, in Newport south wales there's newport which is on the mainline and rogerstone which is on the ebbw valley line. (and you can't get from rogerstone to newport - got to go back to cardiff - very strange). Consequently, a lot of people will make their own way to newport, get a return from there to cardiff then get rogerstone train home (arguing its still newport). Never known anyone getting any issues (probably because last train is rammed full and guard stays put!) but its probably not correct?