Based on your assertion that the table substrate is very structurally solid and your centers are 16" apart or less, then 1/2" ply should be more than adequate and let you climb on it if necessary.
Running on bare plywood can be very noisy, so many people opt for either a 3/4" or 1" foam top glued to the plywood (available at your local big box store in 4' X 8' insulation sheets or a homasote top. In addition, many people run a cork, rubber or foam roadbed directly under the trackage, either separately or in conjunction with either the foam or homasote topper.
Personally, I run Fastrack over 3/4" foam and like the foam because it's easy to work with, takes paint well, and is an excellent medium for inserting accessories like trees, light poles, etc. Not knowing which kind of track you plan on using makes specific recommendations difficult, but you can search the Forum for ideas.
