Truss Bridges
This very common type of bridge features beams that are so large that the top and bottoms are not connected with solid material but by a "web" of bars that form triangular patterns known as a truss. These beams are often connected at the top level to prevent the beams from bending inwards.
There are many different ways that these bars can be assembled, which has led to the many different types of trusses shown below. The most common of these is the warren truss, a design that is still used today.
Railways often used this bridge type because it provided the great strength needed to resists the compressive forces of extremely heavy trains passing overtop of it.
See also arch, beam, cable-stayed, moving, and suspension bridges.