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.