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