Usually a
hot rail is little more than a three-inch
glass pipe that is heated on one end with a
blowtorch. Once this end approaches its melting point, the other end is inserted in the user's nose and the substance to be absorbed is snorted.
As soon as this substance of questionable legality hits the glass, it sublimates and is inhaled as a gas. Since this is effectively a nose-bong, the entirety of said substance is uptaken at once and sprayed on the unsuspecting lung in one hit.
The obvious downside to this is that one is in fact smoking the drug, which can do nasty things (deposition of crystals) to the lungs, as well as tear up the pathway from nose to bronchials -- the amount and temperature doesn't help this process in the least.
Secondly, if one is doing this with meth, the large dose (assuming it is such) radically increases the chance of a heart attack.
Moreover, doing this unarguably qualifies the user as a crackhead, regardless of his or her stance on actual crack cocaine.