I had the same problem with the hopper car of the 29570 set,i kept the wheels,springs etc for spare and throw the body away.Soon after the problem's appearance markln replaced these sets with something elce,usually with another set with similar USA cars.Make a call to your dealer,you never know.

I'm not sure I'd consider that generous, rather, obligatory.

I'm also not sure what your consumer protection laws are like for you but here and Australia are both very similar. Basically goods have to meet some criteria;

1: They have to be "fit for purpose". Ie do what the seller says it will do.
2: They have to last a reasonable amount of time. 'Reasonable' being defined by what most average people buying the same or similar product would expect it to last in normal use.
3: There's also a semi-related factor around goods being of merchantable quality, be able to be repaired and that spare parts must be available for a reasonable period of time.

Simplified wording but basically accurate I believe. I'd be very interested to know what the similar laws are in Germany?

To me, an item that fails due to bad manufacture, materials, or both, while sitting in its original box has not met my expectations. This is one example where buying new from an authorised dealer will be a definite advantage.

Ok, I didn´t provide the full picture: At the point of time I returned them, the items were 8 years old, I hadn´t bought the articles at the MRR shop that supported me (not even in the same country), and I couldn´t produce any proof of purchase. So, I maintain that this was a generous gesture.