Firth of Forth Bridge. Engraving of the Firth of Forth Bridge in Scotland. The bridge spans the estuary of the River Forth at Queensferry. It has a cantilever design, the steel arches being high enough for the masts of sailing ships to pass beneath it. The bridge carries rail traffic. It was designed by Scottish engineer Sir William Arrol (1839-1913), and was opened in 1890. The cantilever bridge design has since been largely superseded by the suspension bridge design.