A cantilever rail is the backbone of a cantilever sliding gate. It is a strong, heavy beam that runs along the bottom of the gate and sticks out past the gate at one end as a tail. That tail balances the gate's weight. It is usually a heavy section like an 80x80x6 SHS, or a maker's own profile from a supplier such as Comunello or Combi-Arialdo, and the tail runs 30 to 50% of the opening width.
The rail is a channel with its open side down. It rides over two roller carriages bolted to a pad or to roller posts beside the opening, on the side the gate slides back to. Because the gate is carried on these carriages, no ground track is needed across the driveway.
CAD60 cantilever models work out the rail length from the opening width plus the tail, set out the carriage centres, and draw the carriages and roller posts at real size.