Try this display of simplicity:
Line up his fleet, side by side. Have him sit in a surrey. Stand behind that surrey. Explain to him that YOU are the "push trailer". Now push the surrey a few feet ahead.
Now he gets out and sits in the next surrey. Stand behind it, then push him a few feet forward.
He'll get the idea how simple, independent and versatile a push trailer will be.
Work smarter, not harder.
LOL, I KNOW you want to modify each surrey, I would, too.
But I wouldn't wanna maintain each vehicle-specific drive system, lol.
"Engine trouble, sir? Okay, let's unhitch it, hook up another trailer and off you go. In the meantime, you can utilize your surrey and I'll work on the ailing push trailer. See you in a few days".