Well, I don’t know if this helps but here is at least an attempt
hier = here if you’re referring to a place you are at. Hier lebe ich, Wir sind hier
da = there if you’re referring to a place you’re not at. Da bin ich (photo), Das Museum ist da
Now a little bit more specific - this explanation does not work in the special case you’re mentioning. I think this use of da is best explained by saying that you’re basically still using it out of the perspective from which you started. Like, you’re leaving the train from Frankfurt to Marseille and you’re still talking from the perspective you’ve had in Frankfurt. Honestly, it’s the only way I found to make any sense of it. Luckily this is not really important - it’s not that natural to say hier sind wir in the scenario I described but you would still be understood.