I think they can be even more clever than that. They could build the carrier in two section the top Carryall, and the passenger cart. After a train pick a full carrier it lower the passengers section to the main train. When it reach the next station the train jettison the empty top Carryall at the same time that it picking up a new full carrier.
from overview for madhi19 http://www.reddit.com/r/gifs/comments/2qllqr/chinese_idea_of_the_train_that_doesnt_stop_to/cn7fwe7