Pretty much the same thing as a heroin shooter. The arm is the 'track', the addict is the 'runner'.
Nurse: He has some small injections between his junk and legs.

DEA agent: He is a track runner.

Nurse: A what?
A: He is a heroin addict.
by akghfaiwjfkasjvkajgk;ae March 18, 2011
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