San Francisco's cable cars are NOT the only mobile U.S. National Monuments.
This is an extremely common misconception. First of all, the cable cars are NOT even a national monument at all! National Monuments are federally protected areas - similar to a National Park, but on a much smaller scale. Some examples of National Monuments are Devils Tower, Fort Sumter, and the Aztec Ruins of New Mexico.
This term is likely being confused with our National Historical Landmarks, which, in addition to the cable cars, include such points of interest as the Empire State Building, the Alamo, and the Hoover Dam. National Historic Landmarks are sites, objects, or structures that have been recognized by the government for being historically significant. Of more than 80,000 places listed on the National Register of Historic Places, less than 2,500 are recognized as National Historic Landmarks. The other problem with this misinformation is that the cable cars are not the only mobile historical landmarks! Besides the cable cars, the list also includes several railroads and ships.
Here's a list of National Historic Landmarks. Here's a list of U.S. National Monuments (which are not the same thing.