I had a similar idea when I first started designing my layout. It seemed so complicated that I abandoned the thought and searched for something else. I found, with even a small amount of space, one could store or stage locomotives on the layout in engine houses.
This engine house sits on a shelf portion of the layout... only 9 inches wide:

The next two houses are on a part of the layout that is only 2 feet wide:


(Need to twist the smokebox front back in place)
For those with lots of room, there are staging tracks where whole trains can be standing by in another room.
Just a thought.
I've been away for a while dealing with an illness. Trying to get back into time for trains.
Regards,
Jonthan