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