single
n.— «Bike trails are commonly known as “singles,” because they constitute narrow trails just wide enough for a single bicycle bearing one rider. Bikers ride in a single file and blend in with nature and the topography surrounding them. » —“Bicycle races are coming your way” by Danit Nitzan Haaretz (Israel) Oct. 30, 2007. (source: Double-Tongued Dictionary)
Further reading
“What has a head like a cat, feet like a cat, a tail like a cat, but isn’t a cat?” Answer: a kitten! A 1948 children’s joke book has lots of these to share with kids. Plus: an easy explanation for the difference between...
The edge of the Grand Canyon. A remote mountaintop. A medieval cathedral. Some places are so mystical you feel like you’re close to another dimension of space and time. There’s a term for such locales: thin places. And: did you ever go...
never heard it called “singles”; usually it is “single track”.
more than you want to know:
<http://www.isu.edu/~nickbenj/bike/mtbdic.htm>
FWIW, here on the west coast we call “fire roads” what people on the east coast call “dual track”.