Hi NUC TUBER,
Yes, Fastrak is loud. I normally run Atlas O track for my Xmas layout. Over the years I ran one 1 loop of track, sometimes 2. Now with Grandkids, more is better, & by the way, noise doesn't usually affect them. I never run on bare wood floors, too noisy. 3 years ago I ran the outer loop with Atlas, the inner loop with Fastrack on a carpet. Still, the Fastrack was almost twice as noisy. I'm not knocking Fastrack, its a nice system, easy to set up & looks nice. If you're having guests over for the holidays & you want to run the trains, you do want the noise level as low as possible so people can enjoy & hear each others conversations. Running slow is a partial solution, but with O gauge trains, track with plastic roadbed is inherent to high noise levels if just used out of the box.
Do you have carpeting in any room for a test? Lay the track there & listen for an improvement. In over 30 years & at least a dozen different Christmas layouts, running any track system on a dense carpet will cut down the noise level considerably. The exercise mats look like a good solution, & would be perfect if covered with white felt available at most craft stores at 72" width to the length you need.
Like I said, I've had many Xmas layouts. The most elaborate was only 5x8 but so heavy it stayed up in our living room till the following Xmas! After the second Xmas, off to the dump.
I've attached an image of last years layout & this years is the same. I believe its 80"x65". The carpet cost $160 because I needed to purchase 12'x6' minimum. Even though the color is "Snow White", its still noticeably grayer than the felt surrounding the tree stand & for me not the perfect solution. The best part is, I roll it up & store it in the attic. It's 3 to 4 times more expensive than the exercise mats & felt though.
I think the spray foam idea could work since it fills that echo chamber roadbed. I'm going to experiment with the floor mats & felt after the holidays. The mats are much lighter & cheaper than the carpet & store easier with the felt, & may be the best solution of all.
I certainly will be interested in your solution.