Manufacturers will build to a drawing, and specification, if the part is not to print, then the fault is with them. however if the part is to print, and still fails, it is down to the design and validation or failing to specify a print which is fit for purpose.
So even if the locomotives are derailing, breaking etc, it is possible that those who made it, are doing what is being asked of them and the specification is in fact wrong.
I must say mind, both of the Duchess' I bought have been fine thus far.
Swings and roundabouts.