[du-bousquetaire]

As Greg said sideways level is most important cause of derailments. I would say worse than bad rail joints at an angle. I once had a twist in the middle of a deep cut (Just as inaccessible as you could want, on a curve, and at a point where you could not make any sight alignement. That is why I made my angle measuring car) This is made from scraps of wood and is useful to measure the inclination on super elevation as well as transverse level.

I had fitted a sprung Märklin SBB box car with a pair of fine scale wheelsets (from Wunder or Dingler) as an experiment, the only place it derailed was exactly at that twist. = result was no more finescale wheelsets, alas!
This car is a sinch to make and most useful doing or adjusting trackwork.