DOUBLE DECK HO SCALE MODEL RR

A friend of mine is designing a double deck HO scale model railroad.

What advice to you have for him relative to building the double deck ?

Thanks.

Doctor *** - The Scenery Doctor

That is a very broad question. The main challenges when building a double deck layout is getting from one level to another and assuring addequate clearance between the lower level and the underside of the upper level.
There are three ways to get trains from one level to another. You could build a mechanically complicated elevator, a space eating helix, or, if the layout is big enough, simply have the track gradually climb as it goes around the room. Each has its pros and cons. I can’t really go into all of them here, but a web search should give you plenty of info.
To determine what clearance you need between levels is a matter of some simple experimentation. You’ll have to make a mock-up to verify the viewing angles of each level. I’ve found that an easy way to experiment is using book shelves that are height adjustable. Keep in mind that typical book shelves are not as deep as typical layout shelf depths. Otherwise, cardboard shelves could be fabricated and duct taped to a wall.