An eagle eyed site visitor has tipped me off that if you change between train and tram at either Elmers End or Wimbledon you do not need to end the rail journey as well as start a tram journey. You must touch in on the tram boarding validator, but if there is an open rail journey on your card this will be resolved before deducting the tram fare. When I called the helpdesk about this they weren’t aware, so it seems as though this is a new feature. I have tested both stations today though and it does work.
Small note. Children using 5-10 zip cards do not need to touch in on the tram boarding validator, but if they have an open rail journey they do need to either resolve it at an entry/exit validator or at a tram boarding validator. They won’t be charged (for the tram) at the tram boarding validator, but the rail journey will still be resolved.