Thanks for all the help, but there are a couple things that might affect the sound deadening solution.
I have a few sound-equipped locomotives and hope to have the rest of my locos equipped with sound in the next year or two. The layout is going to be partially built off the wall and partially supported by a box of 2x4s in the middle of the peninsula. The layout also must be able to be disassembled in sections that are no bigger than about 30" by 5'. I plan to do what was done on the Mississippi Alabama and Gulf (see the latest issue of GMR) and use 1x2 or 1x3 open grid modules over 1x3 stringers atop the brackets. I also plan to have a train elevator (not powered) along the back side of the peninsula.
Naturally using concrete for the roadbed won't work because of weight, but it's interesting because the extreme mass must dampen vibrations.
Rubber isolators between the shelf and wall are a possibility, but the screw into the wall could transmit the vibrations as well as the shelf bracket. If I do put rubber there, then the shelf and backdrop would have to have rubber behind them as well. Good idea - I'll consider it.
Mobilman44, thanks for your suggestion, I didn't plan to have long unsupported sections, but I will use caulk to attach the track, thanks.
Crandel, thanks for your input, though the vinyl and Masonite sounds heavy. How did the drywall work out? The plaster inside must have made it hard to work with... Thanks for the suggestion to use Matte Meduim to adhere my ballast - I'll do that.
Jim, thanks for your suggestion to use homasote roadbed from the California Roadbed Company. Unfortunately, I will need a lot of roadbed, and having it shipped across the country does not seem practical. Thanks for the tip about "hard shell" scenery.
Randy, I'll be interested to hear how the roadbed tests turn out. It stinks that the people at Home Depot don't even know what's being produced - I had the same experience when I asked about homasote!
BroadwayLion, thanks for your input - the subway sounds just like a real one. Thanks for the tip about asking for a lightweight, soft sheet at Home Depot. I got some funny looks though when I asked for some flat cable for my DCC system. I had to rephrase it as "a local network for my train set." I love your subway stations - not many people would go through the trouble of modelling fluorescent tubes that are about to die - AMAZING!!!!
Modeling the Pennsy and loving it!