The Collaborative International Dictionary of English v.0.48:

slip carriage \slip carriage\, slip coach \slip coach\n.
   a railway car at the end of the train; it can be detached
   without stopping the train. [British]

   Syn: slip coach
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

slip carriage
    n 1: a railway car at the end of the train; it can be detached
         without stopping the train [syn: slip coach, slip
         carriage]