This is my first Instructables so I will do my best to include the steps that count towards building your own.
Step 1: The Plan
The room is basically 12' X 14' and my obstacles were a door on one side a window opposite the door with two bookcases on the side and a closet.
I purchased a sheet of 1/2" plywood, I got the better quality plywood so I didn't not have to sand too much. Consulting the NMRA www.nmra.org page I found the minimum clearances and went to my next door neighbor to start cutting on his table saw.
The main loop is 2.5" wide and the part where the main loop and siding are 4.5" wide. Three curves are made to fit 18" radius nickel curves and the third curve with the siding has 22" radius and 18" radius curves. I feel 18" is the minimum radius for a layout because long cars like my Walthers Husky Stack calls for a minimum 24" radius. I have also noted that Locos with 6 wheels or more per truck bind a bit in 18" radius curves slowing the train as it goes around.
I viewed several sites and videos of ledge or shelf mounted railroads and decided that for the level of craftsmanship and tools I had, I had to attach the roadbed to the wall with strips of the same 1/2" plywood cut at 2.5" wide. I was really wanting to use crown molding to dress up the bottom of the shelf and also to help hide any wires but cost, time and my lack of skills made me think twice.