The first rail element 111 is disposed in {HEAD ~ the positive Z direction} right above {TAIL ~ the second rail element} 112 
right above
The first rail element 111 is disposed in { HEAD ~ the positive Z direction } right above { TAIL ~ the second rail element } 112
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH