third rail

from WordNet (r) 3.0 (2006)

third rail
    n 1: a rail through which electric current is supplied to an
         electric locomotive

from The Collaborative International Dictionary of English v.0.48

Third rail \Third rail\ (Electric Railways)
   (a) The third rail used in the third-rail system.
   (b) An electric railway using such a rail. [Colloq.]
       [Webster 1913 Suppl.]