This is a cracked upper drive sprocket. It is from a P&D kit designed to upgrade the Red Caboose GP9. I had this kit sitting on a shelf for perhaps 10 years before opening it up.
Acetyl often shrinks over time and hoop stress can crack a sprocket, universal joint, or even an axle gear. Over many years, stuff happens.
Note: The easiest way to liberate this shaft from the drive tower involves removing the little snap ring shown at the far right. This can prove frustrating without a proper size snap ring plier. If you order replacement parts from P&D, get a few of these rings as well. I eventually found the miniature plier at McMaster-Carr.