Regarding the problem of truck mounting screws removing themselves: I am a big fan of what I call the "Athearn" truck mounting system, where there is a sleeve for the mounting screw to snug up to (sorry about the preposition). On the many models that don't provide such a sleeve, I make my own by cutting off the sleeve on a typical Kadee coupler box. That solution will not last, as my fantasy of using the Sargent couplers may come about.
With the "Athearn" approach, I have NEVER had a screw back out.
I do not understand why people would think a screw would just sort of stay in a hole because it would be a nice idea. There's a reason they're called "screws", after all.