The part of your sentence that says “they refuse to take SOME of the 20 + inch curves on my layout” makes me think several things. +20 inch radius curves in HO are fairly broad so I am surprised you are having problems particularly with the excellent motive power that Kato makes. If the derailments occur in the same places all the time, I would check the track very carefully in those locations for any kinks or twists. If that seems okay, check the track with an NMRA track gauge to ensure the gauge is true in those areas particularly if it is flex-track or hand laid. You can also use the NMRA gauge to check that the boggie wheel spacing is in spec.

If all is well with your track, my next suggestion would be to place some weights on the boggie itself. Try glueing a metal washer or two on the truck (as we call it in the US). It probably won’t take more than two to fix the problem. Paint them flat black and they’ll be all but invisible.

A similar approach would be to mount a small helical spring on top of the truck where the mounting screw attaches the truck to the locomotive frame/body. Either stretch or compress the spring to adjust the ‘down’ force of the spring on the truck. The greater the down force, the better the truck will hug the track but too much will make the truck’s ‘play’ stiffer which may increase derailments. If Kato already has a spring in place on the truck (which is likely), gently stretch the spring to give it more down force.

I can’t help you on part 2 of your question aside from suggesting that you access pictures of the completed turntable on dealer’s sites to get an idea what it looks like when completed.

Hope this helps,

Geof in the US