Do you really need road bed

Directly on the Homesote works fine. Just secure the track to only the homesote. Not into the plywood if you use screws or nails. I don’t think the cork roadbed adds that much as far as making it any quieter with the homesote under it. It’s more to create a ballast profile if your going that route.

If you're putting it on Homasote, other than appearance, you don't really need it.  As for screwing the track down, if you just make sure the screws don't go all the way through the Homasote, that won't be a factor.  The Homasote will hold screws fine, just use short enough screws, probably no more than 3/4".

I hasten to add.  You'll still have plenty of track noise with that Fastrack, short of filling each plastic base with sound deadening material, there's no easy way to quiet that down.  I used to have Fastrack on carpet, it was even noisy there!  Now I have Gargraves & Ross on Homasote and a 1/4" foam roadbed, the difference in sound levels from the track is amazing!  Now I can actually hear all the nifty locomotive sounds that I paid the big bucks for!