737 fuselages are built in Wichita Kansas and are transported by train to Renton, Washington, where the plane is assembled.
Why outsource the fuselage all the way to Kansas? The cost of moving the fuselages by rail can't be trivial. Perhaps it's because of state tax credits, or maybe because Boeing bought Spirit Aerospace which was in Wichita?
Apparently the 737 used to be built in Renton: b737.org