I moved this post from the Train Rules thread because I thought it was a good question. Many people are not familiar with being involved with Coupon Trains, so it's only natural not to know all the "lingo" at first.

"Can't learn if you don't ask", I always say.
A "packing list" (which is what we call it HERE) is just a single sheet of paper which lists the train number (or letter, for Expresses) at the top of the page, and then each riders' name, address and wish list (or list of coupons they're looking for). This paper is sent along with the required 40 coupons from one rider to the next on a coupon train, around in a circle, over and over. It is imperitive that this list be included in the envelope each time a rider sends the train along to the next rider so that each member knows not only who is on the train, and where to send the train next, but what is on each rider's wish list so that they can include what is, well..."wished for". The packing list plus 40 coupons can be sent for the price of one stamp.
This is also the place where riders can make additions, deletions, or other changes to their wish lists - once the train reaches them. If many changes have been made on the sheet, many times a moderator will choose print out a new sheet once it reaches her - just to keep it tidy.
Thanks for asking this question!! I think it's one that maybe alot of newer members may have had!!
If you have any other questions, please feel free to either post it in the Couponing Forum, or PM one of the mods. We'll be glad to help you!!