The Manhattan Bridge, which turns 100 on December 31, flexes under the weight of a subway train in a time-lapse video taken in July of this year. Interestingly, the bridge underwent an 18-year series of repairs ending in 2004, because the subway trains, which use two pairs of tracks on the outside of the lower level of the bridge, made the the structure tilt and sway. This problem worsened as successive generations of trains got longer and heavier. The diagram showsa cross-section of the Manhattan Bridge, which also has pedestrian and bicycle paths as well as seven lanes of roadway on two levels. Enjoy!