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.