If you are talking about the idler gear, they are secured with a flat headed stud pressed into the truck. If the side frame can be removed from the bottom, clean the truck block and stud, add very small amount of super glue in the hole in the truck block, slide the gear between the wheel set and tap back in place. Be sure not get excess glue on the gear.
If the side frame can’t be removed from the bottom, remove the body and then drop the truck block low enough to remove the side frame.
Pics would help.
