I've never seen tubular track with roadbed like that.
In general, tubular track is easy to cut to any size needed (I use a Dremel tool). I try to avoid small filler sections. Let's say that a full length section is 10" long, and I need to fill a 13" space. Instead of a full section and a 3" piece, I'd cut one of the longer length tracks available (usually equivalent to 3 sections) to 13".