The Alco S-4 is a diesel-electric locomotive built by the American Locomotive Company (ALCO) from 1954 to 1966. It features a 1,600 horsepower engine and is designed for switching and short-haul freight service. The S-4 is known for its robust construction and reliability, making it a popular choice among railroads during its production years. With a total of approximately 1,000 units produced, the S-4 was widely used across North America. It is part of the larger family of ALCO locomotives, which includes models like the ALCO RS-1 and ALCO FA. Many S-4 units remain in service today, often repurposed for industrial use.