You could buy a couple of the lightweight plastic saw horses to put you door on. Then complete all of the trackwork, wiring, & scenery on the saw horses. Once the work is done, have someone assist you in putting the door up on top of the Ivar book shelf units. When you complete the second door, & put it up on the shelf unit, then tie them together with some metal plates from your local home supply store, & install joiner tracks and blend the scenery. You will have a little work to do from a stool, but most of the work on the railroad can be completed from a chair.