Summary

I really like the train set thing by KrispyKevin but it has one flaw: the hitches are too simple and come undone very easily, especially if going up the bridge hill.

I set out to design some sort of clip to lock the cars together that would not require remixing/reprinting the actual cars themselves.

This thing is a pressure fit coupler/lock that fits over the connected cars at the hitch point. It fits snug without being too snug, stays in place, and has ridges to assist in getting grip to remove.

If you print standing up you shouldn't need supports.

I have included 2 dimensions of the back block. I think generally, printing the "thinner" version is going to be a good fit for you. But if you find that it's too thin to make good contact, you could try printing the "thicker" version. It has a little more material on it and should fit tighter.