a famous metal railway bridge built in 1889 over the Firth of Forth. People sometimes say that a job is "like painting the Forth Bridge" when they mean that it seems to never end, because the metal bridge takes a long time to paint, and when the job is finished it has to start again.
the Forth Bridge
a road bridge built in 1964 over the Firth of Forth. It is one of the longest suspension bridges in the world.