I think your understanding of “disable” is not correct. Let me illustrate using your scenario.
A - owner of airtag
B - person on train
C- person on train
D - other person who you met after you disembarked from the train
Assuming you power off your phone, the airtag will assume it is not with the owner (A).
After a period of time, both B and C will receive notification that an unknown airtag is tracking them. When they click disable, they are simply disabling the link to their iPhones. It does not mean they can disable the airtag entirely, as they are not the user.
When you disembark and walk past D, the airtag will then hitch onto D’s iPhone. Meaning that you will know that your airtag last known location then, which will be the place you crossed path with D