There is a federal law that governs the carrying of "deadly or dangerous weapons" on federal property, including property leased by the federal government. It does NOT cover carrying them in federal courthouses, at the White House complex, or at the US Capitol and its complex. All of these latter places have their own, stricter, rules. That law is Title 18 of the US Code, Section 930, generally referred to as "18USC930". It says that one may not carry "dangerous or deadly weapons" on federal property, with the exception of those whose duty requires that they do so, but then goes on to define a "pocket knife" with a "blade of less than 2 1/2 inches in length" as not being a "dangerous or deadly weapon." So, you may carry a pocket knife, read folding knife, with a blade of less than 2 1/2 inches on federal property.
There is also a federal law prohibiting the importation and interstate transportation of switchblade knives, gravity knives, and ballistic knives except by or for LEOs and military on duty. This means that most of us cannot legally order such knives through internet dealers. You may have noticed that most of such dealers require some sort of proof of LEO or military status in order to ship you a switchblade. This law is why. It is also why you may see such knives for sale at gun and knife shows with the police making no effort to stop their sales. But do not try to mail one home. Further, I have read that GIs returning from Iraq and Afghanistan are required to divest themselves of any switchblades before they are allowed to return home, although I cannot vouch for that.
