"That is the idea. You will let me know if it doesn't work? The piece of darkness should be pulled towards your target. You'll have to find your way around any obstacles, but it's... something."

There are few that Thanatos can find by sense alone from the train, those strongly connected with death, those having died, Zagreus--of course. The living are harder for him to locate, so he doesn't know if there will be difficulties. It's Thanatos's insecurities that cause him to wonder if this might not work more than the keepsake itself.