Instant roadbed

If you're talking about the asphalt-based product by AMI, it will require a substantial amount of pressure to firmly embed the track into the roadbed, and I'm sure your foam won't survive!

Many folks who have used it say not nearly as well as it will keep every piece of lint and sawdust and dead flies and whatever else happens to fall upon it, except ballast.

I must admit I've never used it myself - one feel of a sample at the LHS brought a myriad of problems to mind, most of which I've heard confirmed in the actual experience of others.