Here is some info on it and this one too
A cantilever bridge just means a bridge that is only supported by one end. They can string them together to make very long bridges. They can be strong enough to carry a train and some are as long as mile in length.
|====================== <--- cantilever section
If you hold a ruler in your end, you just made a section of a cantilever bridge. The top section of the ruler is in tension (wants to separate) while the bottom is compressed. These forces make the bridge possible and balance everything.
An important part is that the center of the bridge is where things meet. There is a lot more to it, but you can read the article to get more info.