If it is derailing in various different places such as on straights and on turnouts and in turns, then the problem is 'likely' the train itself- check the wheel spacings.

I am guessing that it is more likely that there is some issue with the coupling of the coaches.
The other issue could be with the track. M Track that was bent can change the spacing between the rails. The best way to check for this is to ensure that all track sections have the same width and the same profile. If the track is bent (sides pulled in or stretched out, this will cause a bulge or a sag in the space between the rails and can affect the operation of the trains.
One last possibility would be that your track is not quite lined up and the slight shift in the track at the joints is causing the train to derail.

I would check all of these before I started checking the wheel spacing of the model.

Regards

Mike C

If the cars are derailing, it is common (if too much pressure is applied in coupling the cars) for the wheels to be pushed up too far in the boogies. Check the position of the wheels on each car and pull them down, if needed.

r/mark

Make sure all traction tyres are correctly seated.

Are you able to add photos of the areas where it has derailed, or even a short video?