The ALCO S-4 is a diesel-electric locomotive built by the American Locomotive Company (ALCO) from 1948 to 1959. It features a 1,600 horsepower engine and is designed for switching and short-haul freight operations. The S-4 is known for its reliability and versatility, making it a popular choice among railroads during its production period. With a total of about 1,000 units produced, the ALCO S-4 was widely used across North America. Its design includes a distinctive low hood and a robust frame, allowing it to handle various rail yard tasks. Many S-4 locomotives remain in service today, often found in industrial settings or preserved by rail enthusiasts.