The Forth Bridge was built in 1890 on the east coast of Scotland. It is a cantilevered railway bridge across the Firth of Forth, linking Fife and Lothian together. As you can see in the pictures, it is really quite an impressive piece of engineering and design.
Apparently, the bridge has always required constant maintenance. So at least for Scottish people, to allude to the Forth Bridge is to allude to a never ending project. A common expression is, "I'm painting the Forth Bridge," because as soon as the workers finished painting it, they would have to start painting it again.
Sometimes this allusion is used of government legislation when, for example, by the time the legislation gets agreed upon, approved, and passed, it is already outdated and pointless.