I built an overhead track with 3/4" plywood and 3/4" net thick pine boards. I knew it would echo the train sound if I didn't do something. I came up with 2 things to do.
1. 1/4" x 3" or 6 x 75mm closed cell neoprene foam. I laid it under the track. The neoprene foam I used is fire resistent and doesn't absorb moisture. It was cheaper a year ago when I bought it. Still worth it I think.
2. I held the track down with clamps that have slits that fit over the cross ties. Screwing the track down directly can transfer the sound through the screws to the board. I thought this indirect method would minimize sound. The slit in the end slides onto the cross tie. I used black, but this gray one shows better detail in pictures.
Attached to the track.
Clamps shown are made for 4" wide foam. I made shorter ones also for 3" wide foam, they are 1/2" or 12mm shorter. On the overhead no one sees them.
These clamps have a 3/16" or 4.5mm gap under them for the foam. When tightened they push down into the foam 1/16" or 1.5mm. This adds to the sound dedening, not transfereing sound through the clamp. The screw area is outside of the foam. Screws can get wrapped up in the foam and cause problems when scewed through the foam.
I made them to use up to #10 screws. I can't easily find #4 or even #6 screws that won't strip out the head all the time. Too much offshoring...
I am pleased with the result. There is no "track noise" or echos. Excluding the prewar 711 switches. There is a noticeable volume when a steam engine runs over the switch. The switches are echo chambers when a heavy engine runs over them. That is a project for another day.