TL;DR
Same as reading train id of the train currently in the stop, but for train groups.What?
Ability to read train id can be used for filters for trains.Adding/removing wagons changes train ID and forces reconfiguration of the filtering circuit, easy to make a mistake there.
Adding/removing trains from the filters is the same.
This would not happen if we can read the train groups. Just add the train to the group and done.