They do despawn after 24 hours if not killed. We've seen that happen many times, including mid train if there had been no one available to conduct in the last day.

That said, I generally agree with the idea that the massive trains have taken the hunt out of the Hunt for all but those who coordinate the trains. Considering the train complaints have existed for several years now with suggestions on how to break them, I doubt SE is interested in making any changes.

As for players wanting to kill a Rank A they've randomly come across, they should do it. Anyone attempting to shame them for not leaving it to a train is the one who risks trouble with the TOS. A train isn't going to be ruined if 4 or 5 out of 36 are missing - assuming a train even gets run before they've had time to respawn.