In 1963, the American Locomotive Company (ALCO) announced its new Century Series of diesel locomotives. By the end of the 1950s the dieselization of American railroads was essentially complete. For the first time, the locomotive builders had to persuade railroads to replace diesel locomotives, instead of steam locomotives - and because of the rapid pace of American dieselization, few of these locomotives were life-expired. The case for dieselization had been compellingly made - almost too compellingly for the health of the locomotive builders.

In order to win new sales, the builders had to present a compelling argument that new diesels would have a significant advantage over the previous generation of locomotives. ALCO (and most other firms) chose unit reduction as their major sales angle - the offering of higher-horsepower units, powerful enough for three locomotives to do the work of five of the previous generation. Trains were getting heavier, and increasing the numbers of locomotives on a train increases the odds of an on-the road failure with each unit added. Three locomotives automatically require less maintenance than five.

ALCO diesels had done less well than those from General Motors' EMD in the first generation; while they had sold reasonably, nothing had had the success of GM's F-series freight locomotives or their E-series passenger locomotives, or their road-switchers. While poorer reliability wasn't the only cause of poorer sales, ALCO considered it critical to their future success to improve reliability substantially. The Century series featured numerous features aimed at increasing reliability.

Much work was done on the ALCO 251 engine to improve its reliability and reduce internal stresses. A major innovation was the use of a pressurised carbody; all the carbody access doors were sealed, and air could only enter the engine compartment through filters. Filtered air was ducted to the traction motors to cool them; another duct led to the generator compartment to cool the main and auxiliary generators. The waste air from this passed into the engine compartment, pressurising it and preventing outside air and dirt from entering, before exiting through vents in the far end.

At the time of the initial announcement, there were three locomotives in the range. These were the 2000 hp, 4-axle Century 420, the 2400 hp, 4 axle Century 424 and the 2400 hp, 6 axle Century 624. The latter was never built; instead, the initial 6-axle unit was the Century 628 with 2750 hp.

Later additions to the Century range included in the 4-axle range the Century 425 (uprated generator, otherwise as the Century 424), the 3000 hp Century 430 and a large switcher, the Century 415, which was rather unsuccessful.

The 6-axle range added a 3000 hp Century 630, and a 3600 hp Century 636.

Licenses to build most of these locomotives were granted to the Montreal Locomotive Works in Canada and after ALCO ceased locomotive production in 1969 MLW continued producing their versions for many years, including introducing some new models.

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