Each side rail portion includes {HEAD ~ a horizontal wall} between the inner wall and {TAIL ~ the mid wall} 
between
Each side rail portion includes { HEAD ~ a horizontal wall } between the inner wall and { TAIL ~ the mid wall }
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL