Not having any experience with road bed and sub road bed, I spent some time investigating. As with many subjects there were a number of opinions. Homasote may help to reduce noise from rolling stock on track. It may or may not expand with humidly enough got cause track problems. Cork may dry out over time and crumble. Anyway, based on discussion with two professional model railroad builders, I decided to use 3/4” plywood for the bench top. I found some thin vinyl type shelve liner that I thought would help reduce track noise transfer to the plywood.

I performed some experiments with noise transfer to the plywood by temporarily laying some track on the vinyl, cork and plywood. Track directly on the plywood definitely caused more noise than the other two. Interestedly depending on the type of engine that was run the thin vinyl was sometimes better than the cork.

In the experiments the track was not glued to the any of the materials. Later when the track and road bed were glued I found that the glue made a big difference (increase) in the amount of noise transfer. This was disappointing. A good friend that has a large railroad used 1/2” homasote on top of 1/2” plywood. He confirmed that once glue is used noise increases.

Where there is track the vinyl shelve liner is used. If the track section is main line then cork is used over the vinyl.