Since the changes to the routeings of trams about a year ago, and possibly for a time before that, TfL have stated that on any given journey you now only need to touch in on the first tram you use. If you change at Centrale, for instance, from a tram originating at Wimbledon to a tram bound for New Addington you don't need to touch in again. This is in the information provided by TfL surrounding the changes; after all, previously trams ran direct from Wimbledon to New Addington. I suspect TfL deliberately keep this information otherwise secret, knowing that if people touch in again they'll be charged again, subject to daily caps of course.