{HEAD ~ a third rail}, oriented perpendicular to {TAIL ~ said first and second rails}, and slidably mounted to said first and second rails 
oriented perpendicular to
{ HEAD ~ a third rail } , oriented perpendicular to { TAIL ~ said first and second rails } , and slidably mounted to said first and second rails
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH