ALCo built a total of 131 Century 420 locomotives between 1963 and 1969, when the builder ceased all new locomotive production.
Powered by a 12-cylinder, turbocharged, 2,000-hp 251-series prime mover, the C420’s direct competitor in 1963 was the EMD GP18. In fact, EMD did not offer a 12-cylinder, 2,000-hp prime mover until the GP39 model was produced in 1969. The shorter 12-cylinder engine block allowed the C420 to have its distinctive set-back cab and extended short hood.
The first road to purchase the C420 was the Lehigh & Hudson River, with its first two units built in 1963. The largest fleet was purchased by the Long Island Railroad, with 30 units built between 1963 and 1968. All were equipped with a high short hood which housed a steam generator for passenger service. Over time, the largest fleet of C420s was amassed by the Louisville & Nashville. While only 26 units were purchased new, their total fleet grew to well over 60 units through mergers and acquisitions. The C420 can still be found in daily service today in the US. Currently the largest fleet of C420s is operated by the Arkansas & Missouri Railroad.