George,
The number is 718, and it's a mail pick-up car. It was molded in green or red plastic. It's one of the most reliable and pleasing animated cars in the Flyer line-up. When activated, it picks up and THROWS out a mail bag so fast, it's hard to see the process. That's why it's recommended to use two different colored bags!
There also is a 732 baggage car that ejects a small "crate" from an opening door. It uses the same mechanism that the operating boxcar IIRC.