It takes less than half a day to change wheels.
Sorry, but I don't understand how that fits in with the possibility of making only a one or no stop trip from Xian to Hungry or Yixinou to Madrid. The latter trip requires 3 stops to shift the load onto different wagons because of incompatibility of track gauges.
Most modern tracks are monitored electronically but as far as I know because I actually saw them at work in parts of Russia and Eastern Europe they still employed "wheel tappers." When the train is stopped they walk along the length of the train tapping the wheels with a bar. A wheel with a crack makes a different sound to a wheel that hasn't. So I guess along a trip that involves less modern tracks and can take as long as three weeks provides another reason to shift the load onto another set of pre checked wagons where the gauge changes.