a laser diode having a beam situated at {HEAD ~ one end} of {TAIL ~ said third rail} 
of
a laser diode having a beam situated at { HEAD ~ one end } of { TAIL ~ said third rail }
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL