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