What goes up, must come down

I love these Scherzer Rolling Lift Bridges by North Station. There used to be 4 at this location, when rail use was higher, but they are down to 2 now. And I heard the T was planning to replace them, which is a shame. I believe they were built in 1931.

You can get a good view of them opening and closing (and really see them "roll") from the North Bank Bridge, which is under the Zakim Bridge between the North Point Park in Cambridge and the Paul Revere Park in Charlestown.

An interesting history of the bridge type can be read here:

http://www.entechcreative.com/images/WhitePapers/Scherzer-White-Paper.pdf