Radar observations of a meteoroid entering the atmosphere occasionally exhibit a rapidly decaying echo which appears to come from, and move with, the region of the meteoroid itself. This head echo usually accompanies the normal meteor radar echo which is essentially stationary and may persist for several tens of seconds. The problem of the radar meteor head echo has remained unsolved for many years. This paper considers the possibility that the key to the enigma may lie in the very high recombination coefficients of molecular cluster ions and concludes that ionized trains containing a sufficient proportion of water cluster ions should exhibit this type of echo. The source of the molecular clusters may be either atmospheric or meteoric but the observations do not yet allow us to choose between these.