Well, we do this with passenger trains as well. American trains share track with with heavy freight trains (and might collide with), and so are built to much different standards than European passenger trains that have dedicated tracks (or new exclusive use while they are running).

The German rail system is also built with the assumption of tracks being shared by passenger and freight trains, with very few exceptions.

Trains are generally not really designed to collide with anything at all (except maybe the occasional suicidal deer or human, but I doubt that this requires any particular design considerations), certainly not "train A is only designed to deal with stray passenger cars whereas train B should be able to deal with heavy freight".

What might be a design issue, more for the network and operation procedures than for the rolling stock is confidence about not meeting anything like a stray car. Different approaches exist, including train-side boxes on the last car or axle counters in the network.

> Trains are generally not really designed to collide with anything at all

American trains seem to suffer this more than others. Given that freight trains have priority on most of America's rail lines (which the freight companies own), that might have something to do with it.