Someone else demonstrating the car-trimming mechanic was enough, I think. Frustrating that it isn't tutorialized in the actual game - I solved several engineering problems that simply did not do the job because I was guessing wrong at what I needed to do - but I appreciate the walkthrough you're presumably giving me behind that spoiler.
Basically to summarize without anything else, because I will not re-encode what I’ve said there, is there are a lot of oddities. I think a number of them are not working as supposedly in their intended behavior, and would probably need to be reworked and have a new level to introduce the behavior. Tho these tiny block pusher game series they make were always made in procrastination, from what I witnessed, so bugs/unintended behaviors and/or solutions are bound to happen.
I tried to adjust the level so that you wouldn't be able to solve it by making the carts crash via a misaligned track (didn't realize that was still possible), the intended solution is (spoilers) using the pushable box to get the carts on one of the trains to crash without crashing the train itself.
Yeah, that much of the solution was obvious, but I still have no idea how. Pushing a box into a cart doesn't do it. Crushing a box between carts doesn't do it. The box doesn't push carts, so I can't push one into a wall. There isn't a convenient turn-right-before-a-wall implied by the architecture to let me push a box into the way of a turn after the train has passed, though I think I can see how to make that not necessary now.
(I never saw the point of the fourth of the three hollow cubes, by the way. Not sure what the 'easy' path was.)