Noise

Unscrew from the plywood to start. Put something soft under the track like Homosote, foam, indoor carpet. It will help.

Bro, you need insulation of some kind as mentioned above. :-)

Temporary? Have some fun with that fact and throw a rug on it  Especially funny if it's just an oval on a nice oval throw rug (I used a soft "spiral rope rug" on a table outside and it was near silent and looked the part. A rag rug would be equally "home-y" imo.)

You noise is comming from different sources too, reflection off the top; transmission to the layout/"drumming", and it's echo from the underneath. Not screwing track down tight, isolation layers (foam/homasote), and heavier skirt curtains help with the underside noise. Top reflection will be based on your scenery and ground cover. Smooth and hard will be louder, static grass and soft moss bushes everwhere will tone it down.

   Carpet is actually one of the best sound insulators. Comming up off the floor you'd expect might quiet it some, but it is the opposite.

Cross braces on the frame help with drumming too.

. If you aren't too worried about realism indoor/outdoor AstroTurf is great and hardly allows any shifting. My pal used it and I wasn't keen on the idea till I saw and heard it. A short nap rubber backed resteraunt/bar carpet would likely be best though. Just enough nap to prevent shifting, and short enough loose fibers arent wrapping tour axles. (high quality will help on loose fibers too) 

Pin your track from shifting vs screwing down tight. Tight screws transmit more vibration to the wood. Or use small amounts of caulk to glue it. Nylon hardware is another option. Rubber washers would help some if you must screw it down tight.

Foam works, but I was kinda expecting it to work better. For below grade/ terrain carving it cant be beat beat though; makes it very easy and carving mountains, etc. you almost can't mess it up bad carving or painting....it's that easy.

  I've wanted to experiment with no nap carpet sandwiched between plywoods or foam. I think that would just leave the sounds reflecting off the top.

If you do use foam as a sub-roadbed, you can try zip-ties to hold down track. That's all I use on my permanent layout. You can use them on plywood also. 

Plastic zip-ties instead of screws will be a big improvement.

Update I got some 1/2 simi hard packing foam and made a road bed under some of the track more quiet. I'm not to sure about Zip ties to hold it in place. Layout will be stored on edge when I'm not using it.it seems to me screws would hold it more in place than  Zip ties due to it having 5 switches on it.

you thoughts please,

Jess

P.S.   Here is my first photo way before foam

Never got your photo. I would love to see it. By the way, I think that zip ties, properly used are just about as tight as screws. Here's what I do. I drill a hole in the railroad tie and then down through the roadbed and sub-roadbed (in my case I use cork over insulation foam over plywood) just large enough to accept a thin zip tie. Then I stick the zip tie (#1) down through with the nob (or whatever it's called) stopped by the railroad tie. Then from underneath the sub-roadbed I slide another zip-tie (#2) onto the one (#1) that is protruding from the sub-roadbed. Then with a pair of pliers I hold onto the nob of Zip tie #2, while pulling zip-tie #1 as tight as I can with another pair of pliers. Of course the pliers are optional, but they make the tightening much easier for me anyway. One point that I learned from trial-and -error is to make sure that the hole you drill in the tie is not too close to the outside rail. Otherwise, the nob will interfere with the flange on some of the freight cars. I'm sure all of the above is fairly obvious, but it took me awhile to figure it all out, so I thought I'd share it. Doing it this way has proven to work well and provide a very tight fitting track bed. By the way, doing it  this way I have never even glued my cork roadbed down, since the zip-ties also hold it.

As an aside comment.  Nothing is as silent as solid two rail 0 scale trains.  Noise is produced by movement and friction.  Eliminate/reduce  movement will attenuate sound.  Once sound is produced then anything hollow or semi hollow plus hard surfaces may increase db.

I laid my track on cheap carpet squares.  I also screwed the track down through the carpet squares to the plywood too. But, I found that if I didn't screw them absolutely tight, the noise level was cut down considerably.  And, the track doesn't need to be screwed tightly.  Just enough to keep it from moving and separating at the joints.  Sure, there is still track noise and the clickety-clack, but it is tolerable.