The bogies and underframes are well made out of welded mild steel. The wheels are pressed onto the axles but inside plastic bushes, as remember the original TMNR was a live two rail pickup system, so each wheel needed to be insulated.
The axle ends fitted, with a spring and washer, into a nylon or similar plastic axle box. This in turn was spring into the bogies frame and held in place with a bolted down metal clip.
When new this probably worked very well, but over time, especially when used by Showmen or commercially, dirt soon got into the axle boxes and soon started to wear away the plastic, resulting in oval end slots!