Adding Snow to Fastrack

I am curious about this too.

Baking soda looks really nice.  However I do not know how to fix it so it does not blow around.

Seconded!

Fibrous and loose material will inevitably make it into the gears and drives of our beloved engines.

Material that is thick will prevent the wheels from running and requires a lot of pruning to make sure wheels fit.

White electrical tape is an option, but you will have to deal with the adhesives left behind. Its also glossy, so that might be a turn-off for some.

Flat material may look aesthetically unpleasing, but it might do the trick between the tracks. Low-temp Hot Glue is the least destructive glue I can think of, but will satisfy your requirement of not being permanent. There are also types of rubber cement and silicone that dont bond to materials, just hold them. Maybe latex caulking.

I'm thinking strips of copy paper cut down to fit between rails, and tacked with very small drops of adhesive in various places could do the job nicely.