Put the meeting after the delayed arrival
If the later train arrives at 3:00, allow enough time to find the exit, deal with luggage, and reach a clear meeting point. Meeting at a fixed café at 3:40 is usually easier to communicate than asking someone to catch up with a group that keeps moving through a museum.
Keep the first group's plans independent
Check where keys, admission tickets, bags and contact details will be. A key on the later train means the early group cannot promise to leave luggage at the rental unless another access arrangement is confirmed. Choose an early activity that works with the bags people will actually have.
Agree on one update route
If the arrival slips again, send one updated meeting point instead of a stream of changing locations. Decide who will watch for the message and who can continue enjoying the outing.
If the train is delayed until 4:30, have one person send a new meeting place and time that will stay fixed until then. Tell the late traveler whether they should eat on the way or expect to join the group for dinner. A practical detail like that is more helpful than a series of photos showing where the group used to be.