{HEAD ~ means} for moving {TAIL ~ said third rail} along said first and second rails; and 
for moving
{ HEAD ~ means } for moving { TAIL ~ said third rail } along said first and second rails ; and
HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH