Foam is simple and easy, but I didn't get the result I expected. Nice?, Sure. And for more than just flat terrain, a godsend. I don't find the plastic roadbed tracks any quieter than tubular, the sounds are just at a different frequency (and I'm not fond of roadbed noise's higher pitch)
I never thought moving to a layout from carpet would be quite so loud. I had been around plenty of basement layouts before, but doing in a normal room the noise becomes more apparent. Sometimes perception is a bear.
The flat out quietest I ever heard was felt, carpet, homasote, caulk, and cork alone. In that order and finding big felt isn't easy anymore.. Added layers when soundproofing help a lot too. (hard/soft/hard/soft, etc. hard blocks & air gaps absorbs)
Also, unless you plan on moving it, it is better to pin track from shifting than hold it down tight; Nylon hardware helps stop vibration trasfer too.
For a cheap underside material, if you were inclined, old egg cartons stapled to the underside. I've used them for soundproofing low budget band studios a number of times; sometimes layered near a foot thick 
When they get in the way of a rewire, etc. rip a few off, and go make an omelette after the fix 
