In Marx’s cheapest sets, it utilized cars that had sliding couplers with only a twist holding them in place. The end of the coupler had a couple of tabs, and a twist secured them without any additional parts.
The problem is, sometimes this twist catches the slot wrong when going around a turn and gets stuck. Then the cars derail as the train comes out of the curve. That’s probably not what you want.
Here’s a cheap, easily reversible modification to keep them from hanging up.
Take a small electrical zip-tie and wrap it around the indentation right in front of the tabs. Secure it tightly into the indentation, then snip off the end. The zip tie is larger than the slot, so it limits movement to the point where the coupler can’t get stuck anymore.
The nice thing about this method is it’s non permanent.
If you ever want to remove the zip tie and return the car to its original state, simply cut the tie with a pair of diagonal cutters, or grab the head of the tie with a pair of needlenose pliers and twist until it snaps.
I’ve seen people secure the couplers with screws to keep this from happening, but that’s a permanent modification. If a car is in nice shape, I’d rather not modify it permanently. Plus, the zip-tie trick only takes a minute and only requires one hand tool.