A NAIL is a RAIL of cocaine that has a large bump at the beginning(or ending) point, thus creating a large line resembling
the likes of a... nail.
One may decide to utilize
the bump at the beginning for an
explosive start to their coke binge or at the end as a "grand finale".
The fatness of this "bump" (or "nail head") may be 1.5 to 2.5 times that of the primary rail.