Replying to Modular systems?

I think I made a good idea for attaching separate rails together. I'll draw up something, or take pics eventually. But how about an idea to change the rail on top of a support structure, like from double to singe, pvc to metal? something like that. I'm trying to think how that would work. I was thinking maybe using hinges or something so i could repeatedly screw and unscrew a rail from the top. the rail would be attached the the support on the sides. i'm gonna draw something and post back.

ideas anyone?