Hello CSXRAILFAN, glad you like the elevated layout me and some buddy's built over a weekend (although I must admit, it really helped that one was a master carpenter...me and the other 2 just did as we were told, haha). Its still up and working great after about 4 years already. Lumber is pine 1 x 10's for the vertical backing that is attached to the wall. The bottom bed is also 1 x 10's, which is butted to the back board. Upper bed is 1 x 4's, also butted to the back board and supported by 4" x 4" squares made of 1 x 4 lumber attached to the bottom bed, vertical back board, and upper bed. The whole thing was built as several modular pre-assembled 8, 6, or 4' spans that were then secured to the wall like an interlocking puzzle (back board and bottom bed boards were alternately offset by an inch to interlock to provide sturdiness). It's really a very sturdy 2-level shelf. The corners were made from 2' x 4' x 1" pine panels (not plywood) with the curves cut to match the radius of the O-gauge track. The top track (outer) has a larger radius track than the lower track (inner).
The one thing I would change if I did it again would be to increase the clearance between the ceiling and the upper bed. When we first came up with the dimensions, the tallest engine I owned was a Lionel Post-war F-3 Diesel Superchief. I figured that would be the tallest engine I'd ever own, but as this hobby grew (remarkable how fast an engine collection grows after you have a permanent layout built), I managed to obtain taller engines that do not fit on the upper track (they scrape the ceiling). It also makes it tricky to add smoke fluid to engines on the upper track, but still can be done. I would have perhaps used only 3.5" tall supports under the upper bed, as opposed to 4" tall supports. This would have given me an extra 1/2" of clearance, but would still be elevated enough to see the upper train behind the lower train. I never planned on it, but this hobby has a way of doing this: I eventually got myself an a few LionChief radio controlled engines, then an MTH DCS control system, which enabled me to run multiple trains on both tracks. I usually run two trains on each track: 2 passenger trains on the upper track, and two freight trains on the lower track. I must admit, one night I had 3 trains on each track (6 trains total)...that was fun, but VERY stressful as the LionChiefs don't have quite the speed control as the MTH DCS controlled engines...it was ALOT of remote control juggling, but super fun! Enjoy!
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Not to mention the Let's Go Pens poster. 
