Charcoal-gray carpet is a good choice (I'm not saying that because that's what I used, lol):
It provides a nice contrast for colorful tinplate trains, and has a "ballast" look about it.
Now, I did screw my track down to the table (constructed of 3/4" plywood on a 1x4 framework, covered by gray indoor/outdoor carpet). I found that if you don't attach the track it tends to scoot around the table-top over time which can be problematic if you have any significant scenic work. Screwing it down will, however, increase the amount of noise that's generated by the trains running on the table-top (sort of a drum-head effect). An alternative might be to use zip-ties to hold it down...being less rigid and of a softer material will lessen the sound/vibration transmission.
Some guys apply ballast to their track-bed. The glue/ballast combo will actually hold the track in place horizontally. I don't think that's an option with a carpeted table-top however.
Personally I like the racket the trains make running around the layout.
The backdrop looks great Howard...looking forward to seeing more pictures as the project continues.