The one you get for your settlements from doing some quests for the people at Bunker Hill.

Had one playthrough where I unlocked this, set it up at Sanctuary and it worked fine -- usually in the daylight hours, one of the traders from Bunker Hill would always be there.

On this playthrough I've set up at Red Rocket and have unlocked and placed it there. Nobody is arriving. I've got it where there's plenty of space for traders and their Brahmin, but none of them ever show. I've hung around the settlement during the day doing building, slept for an hour or two to see if anyone would show up, scrapped it and built a new one, nothing.

Does this only work at Sanctuary then? Has anyone had any luck getting traders to show using it anywhere else?