Homasote?

It's still used today, tougher to find in some areas, but still around. It seems to help when laid over OSB or plywood. In fact I wouldn't use it without support

My feeling is that you have to build up mass to eliminate vibration which translates to noise.  Placing plywood over benchwork can create vibrations that you have control with substances like homasote sandwiched on top.  The problem I have with homasote is the difficulty for it to hold screws.  I would like my train to sound like it was running on top of a concrete floor with some carpeting underneath the tracks.  The only way to do this effectively is to use drywall as underlayment to build up mass (ie. weight).  I would place drywall on the bottom, plywood on top, and then use cork over the entire surface (which is good for self-mending screw holes if you want to change anything later on).  This has been successful in many instances, although I recognize that its weight can be prohibitive for some people.

As Chris said I would not use with out the OSB, plywood support.

I'm rebuilding and used information from a lot of post by members.

I used a product called Quite brace sound proofing. Much cheaper and homasote is not available in my area. Quite brace is found in the home stores. It is some what messy and it's recommended a coat of cheap latex paint be applied. Preferable a base color of your choice to match future ground cover.  I used a light green and glued the quite brace to the plywood.

Also it suggested when you attach anything to the finished top (soundproofing) don't let the screws go into the plywood/OSB wood. (reference noise reduction)

Yes, I think it makes a difference, especially since I used about 200+ feet of fastrack.

It has a reputation of being the loudest track.

good luck with your project.

Larry