Ft Knox doesn't have the largest store of Gold in the US.
The largest is in the Federal Reserve Bank in NYC! Not only the majority of the US gold, but believe it or not, a LOT of other countries have a supply of gold there. A lot of "gold transfers" are done by moving bars of gold from one cage to another. Also, many of the major bank HQs in the area have gold vaults - fun to watch Chase or the Bank of Tokyo getting a gold shipment (or the other way) - picture an armored cars that is a tractor trailer. The pull up outside than bank, and a forklift comes out and unloads the truck. Needless to say, there are armed guards in the area. Usually with machine guns. What is particularly fun is that 99% of the people walking by don't stop to look - after all, we're NYers, so we've seen everything