Best way to affix (code 80) n scale track to grass mat over (pink) foam?
I would not place the track on the mat. It will be quite spongy and your loco weight will cause it to run perpetually in a slight dip as it moves from place to place. Can you not do what most of us would do, situate a roadbed of either cork or foam, maybe the eco-cork cut to shape and length that Home Depot sells in rolls? Establish your track on that material, with the roadbed of choice thinly caulked to the surface below it. Then, once the rails are in place, cut and trim the grass matt to meet up to the rails. Your tracks will be properly supported, and you won't have the tricky matter of finding a glue that will work.
If you are absolutely committed to what you have described, I would use something I could spread that would also embed itself into the many filaments in the matting. I would spread spackle or drywall filler, try to keep it on a constant grade, and then place the track on it. Maybe lightly glued with DAP Alex Plus with silicone. The idea would be to lay the track segments before the compound sets up so that you can get it to lie smoothly.
Maybe you'll get better ideas, but I really do have serious reservations about laying tracks, even EZ-Track or Unitrack type segments, directly on a mat.
What holds the mat to the foam?
What holds the Track?
Nails do not work well in foam, glue does not work well on a mat.
LION does mount tracks directly to foam with nails, but then runs a bead of glue over the tracks to finish the job. The mat, if that is what you want to use, (I'd rather paint the foam and use ground cover) should be cut and fitted into place.
ROAR
The Route of the Broadway Lion The Largest Subway Layout in North Dakota.
Here there be cats. LIONS with CAMERAS