Many bridges cross rivers and other bodies of water. Boats passing under a bridge are usually moving pretty slow (compared to trains), but boats have incredible mass. This means that even a barge, which typically creeps along at very slow speeds, can impart tremendous force if it collides with bridge pilings or piers. That force is sufficient to knock down the bridge in some cases.
An example of this type of incident is the collapse of the Judge William Seeber Bridge in New Orleans in 1993. The bridge carried road traffic over a canal, and a barge passing under the bridge struck a pier supporting the bridge and severed it. Nearly 150 feet (46 meters) of bridge collapsed as a result. One motorist driving on the bridge at the time died in the accident [source: NTSB]. More than a dozen major bridge collapses have been caused by boat collisions in the last 100 years [source: Wardhana].