The ALCO RS-3 is a 1,600 hp (1.2 MW), B-B diesel-electric locomotive. It was manufactured by American Locomotive Company (ALCO) and ALCO's subsidiary Montreal Locomotive Works (MLW) from May 1950 to August 1956, and 1,418 were produced — 1,265 for American railroads. It has a single, 12 cylinder, model 244 engine. The RS-3 was the successor to the RS-2.   The RS-3 was an evolution of ALCO's preceding RS-2 and RS-1 designs, which are both very similar to the RS-3 in appearance. Compared to the previous RS-2, the RS-3 differed by increasing power output from 1,500 hp to 1,600 hp, and in some changes to the fuel system and body shape. In most aspects, both models were identical, using the same ALCO 244 prime mover and hood unit design.

ALCO built the RS-3 to compete with EMD, Fairbanks-Morse, and Baldwin Locomotive Works. In 1949, EMD introduced the EMD GP7. In 1950, Fairbanks-Morse introduced the 1,600 hp H-16-44. Also in 1950, Baldwin introduced the 1,600 hp Baldwin AS-16. In the case of ALCO, Fairbanks-Morse, and Baldwin, each company increased the power of an existing locomotive line from 1,500 to 1,600 hp, and added more improvements to create new locomotive lines. All of this was to be more competitive with EMD. ALCO's 1,500 hp  line was the RS-2, although 31 were built in 1950 with 1,600 hp. Fairbanks-Morse's 1,500 hp line was the H-15-44. Baldwin's 1,500 hp  line was the Baldwin DRS-4-4-1500. EMD, however, kept its competing GP7 at 1,500 hp. But in 1954, EMD introduced the GP9. It was rated at 1,750 hp.

In the end, EMD won the road switcher production race. EMD produced 2,729 GP7s. ALCO produced 377 RS-2s, and 1,418 RS-3s. Fairbanks-Morse produced 30 H-15-44s, and 296 H-16-44s. Baldwin produced 32 DRS-4-4-15s, and 127 AS-16s.