"I haven't figured out how the train tells the difference, or if it's even the train, really," Nita answered, picking up one of the boxes to carry back to the Stores carriage.

"But if you think they should have come with you, but they haven't, they'll probably have to be brought in on their own."

Nita blinked, wondering where to start.

"There can be a lot of overlap - and when you can talk to the world, you get to see a little more of the picture than most - but how it all fits together varies a lot world-by-world."