I think I would use "cassettes" with tracks for staged trains. You could then build a small cart that the cassettes would be set on top of that would then line up with a track on the main layout. You could actually have more than one cassette stacked on brackets under the layout and then interchange them on the cart for different consists. I think I would also make some sort of "draw bridge" to bridge over to the cart and cassettes so that it could be stored in the "up" position to keep trains from taking the "big drop" to the floor when the cassette is not in position.