Carpet underpadding over plywood

No, but I have used it sandwiched between plywood sheets as an insulator in an experiment and it doesn't work there at all.  Since you have fastrack that many feel is noisy, why not put some non-flammable fiber-fill inside and then use the carpet underpadding to isolate the vibration from the plastic bottom of the fastrack touching the plywood.  The only problem is that it is difficult to fix the track down through the padding without going into the plywood which will induce vibration and sound transmission.  Given the fact that the under padding is really not that rigid, after awhile it may have a tendency to ride in the direction of the train, especially on curves, so you will have to stabilize it with glue on the plywood and also fix the tracks through it   somehow.  In my experiment with it between the plywood, I had Gargraves track fixed to the top layer of plywood.  There was carpet padding underneath and a sub-base.  As my train went around and around, gradually the padding worked its way to the edge of the table on the outside (the padding was pieced in the middle of the board and not fixed with glue--just gravity).  So it can move.

Thank you. 

I am considering double faced tape in a few places to preclude the carpet from shifting.  An advantage of the Stainmaster brand underpadding it the very thin backing which takes liquid and aerosol paints without cracking or peeling, no matter how sharply it is bent over.

I may just go staight to some heavier cut or loop pile carpet or perhaps Berber.

I can understand what you want to achieve, but the reduction of sound transmission is always a matter of compromises.  Some people believe that using dry wall board under plywood makes the platform too heavy.  It does, no question about it.  But it effectively increases the mass of the platform to stifle vibration, much like running a train on a concrete floor.  You have two different problems:  one is the vibration and sound transmission from the platform stemming from the train tracks on that platform that induce the vibration from the train itself--the tracks being the conduit.  The other is any sound transmitted into the air from the train just going on the track sections.  Some have used rigid foam, homosote, carpet, cork, etc. to absorb (but not eliminate) vibration.   Different densities of these various materials are more or less effective for this.  I would prefer increasing mass first and then applying any of those materials.  For a compromise, however, you may just want to place the carpet down over the entire layout (it will increase the mass because carpet is heavier than rigid foam or homosote at the same time it has some density from the piles to absorb sound from the tracks).  I used cork myself in this fashion simply because I was able to fix screws into the track and cork without going through into the plywood.  Carpet will not allow for this so you might have to screw the track past the carpet into the wood.  The compromise:  Use fewer, more strategically placed screws.  The carpet can absorb some vibration transmitted from the screws into the plywood--so it won't be totally sound-proof, but covering the entire board will help as well.  Sometimes even though vibration is not reduced enough, the materials it passes through causes the frequency response of the sound caused by that vibration to be lowered so as to be inaudible or below the threshold of one's offense to the sound that is generated.

Thank you as well. 

The layout is not large - 6 1/2 x 12 and the plan to apply carpet or underpadding over the whole table.  Just need to decide which material to use.  Testing with underpadding (6' widths much better needless to say than staight plywood.  Carpet (cut pile, 40 oz/yd) better still.

Monday is decision day.  Again thanks for your intertest and information

My layout started as a carpet layout on steroids. Built sections of tabletop without legs to sit on floor. Framed with 1x2 with 1/2 ply. On that i used indoor/outdoor carpet in green. Found the right stuff at Menards. Had rubber backing and no grain. Some have a definite pattern.  At that time they also had utility/trunk liner in various colors. On top of this I used Homasote cut to shape and beveled for profile with the same carpet in grey to represent the gravel/ballast.  Green carpet was glued and stapled to plywood. Homasote was screwed to plywood. Grey carpet was stapled to Homasote. Atlas track was screwed to Homasote. Eventually the layout was raised to 42 inches and filled the living room.

The whole thing was very quiet.