Hills, while the Native American Scenic Byway runs along the Missouri River in the north-central part of the state. Other scenic byways include the Badlands Loop Scenic Byway, the Spearfish Canyon Scenic Byway, and the Wildlife Loop Road Scenic Byway. Railroads have played an important role in South Dakota transportation since the mid-19th century. Some of railroad track were built in South Dakota during the late 19th century and early 20th century, but only are active. BNSF Railway is the largest railroad in South Dakota; the Rapid City, Pierre and Eastern Railroad (formerly the Dakota, Minnesota and Eastern) is the state's other major carrier. Other state carriers include Dakota Southern Railway, Dakota and Iowa Railroad, Ellis and Eastern Railroad, Sunflour Railroad, Canadian Pacific Railway, and the Sisseton Milbank Railroad. Rail transportation in the state is mostly freight, but there are two passenger heritage railroads: the Black Hills Central and the Prairie Village, Herman, and Milwaukee. However, South Dakota is one of the two contiguous states that lack Amtrak service. (South Dakota is the only contiguous state that never had Amtrak—Wyoming used to be served by the San Francisco Zephyr and the Pioneer.) South Dakota's largest commercial airports in terms of passenger traffic are the Sioux Falls Regional Airport and Rapid City Regional Airport. Delta Air Lines, Frontier Airlines, and Allegiant Airlines, as well as commuter airlines using the brand affiliation with major airlines serve the two largest airports. Several other cities in the state also have commercial air service: Aberdeen Regional Airport, Pierre Regional Airport, and Watertown Regional Airport, some of which are subsidized by the Essential Air Service program. Public transit played a large role in the development of cities in South Dakota. There were seven cities with a streetcar system in the nineteenth and twentieth centuries, however, all of these were discontinued over time. Today, only three fixed route public transit systems exist in the state, those being in Sioux Falls, Rapid City and on the Yankton Reservation. Government and politics Government Like other U.S. states, the structure of the government of South Dakota follows the same separation of powers as the federal government, with executive, legislative, and judicial branches. The structure of the state government is laid out in the Constitution of South Dakota, the highest law in the state. The constitution may be amended by a majority vote of both houses of the legislature, or by