This is an improved version of an 00/HO scale chassis for UK coal wagons and covered vans. It uses brass bearings and metal Dapol wheels to ensure good rotation..

glue "top hat" brass bearings (00 Gauge Markits Pin-Point Brass Bearings, Shouldered Type) into holes on inside of chassis.
Glue one half-chassis to edge of underside of body
Glue other side and, before glue has set, insert wheels (Dapol spoked) between two sides. Check for smooth rotation.
Add brake rods (see pic). Paint with acrylics and weather with powdered crayons