caisson: [kā-sän] noun. A watertight chamber used in construction work under water or as a foundation.
The pieces of equipment in the photo above are building caissons for the train bridges that will be part of the roof of the bus terminal. (I had to look up the definition of a caisson.) There will be eight sets of tracks (Amtrak, Ski Train, commuter rail) requiring four bridges. As we know, trains can be pretty heavy so the bridges will require deep, sturdy foundations. Therefore, the caissons.