My gondolas usually carry Lego people or whatever the plushie toys the kids are playing with at that moment. One of the common sayings down stairs is "Stop the train" - as said in the polar express. that way they can have their little friends get on or off.
I think marbles is a great idea, and that loader is cool! For these weeks though, I would go with jelly beans. I am sure someone will soon post the Marx trains Easter video showing the trains carrying candy.