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
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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”.