The supporting element is slidably disposed between {TAIL ~ the first rail element} and the second rail element along {HEAD ~ the rail} 
The supporting element is slidably disposed between { TAIL ~ the first rail element } and the second rail element along { HEAD ~ the rail }
OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD