The Saint Michele bridge. Paderno d'Adda (Italy)
The Cassandra Crossing bridge seems to be the one which was built here, in Italy by engineer Jules Rothlisberger between 1887 and 1889. It's located in Lombardia, near Milan. In this forum, I've read other topics about a very similar bridge in France, but since I was a child I've known the italian was the only one in the movie.
Here you are:
http://www.belsit.com/il_ponte.html