Each driver consists of a cast-iron or cast-steel wheel disk onto which a higher-quality steel tire is shrunk. Mainline passenger locomotives can have drivers up to 90 inches in diameter. The K-28s, with their tiny drivers (44-inch diameter), were built with intentions as freight locomotives, but they served as passenger engines because they were lighter than the frieght-oriented K-36s and K-37s (and, because they pulled less weight, were also almost always available).