A six-axle variant with the model number 628 was already among the first models that ALCO offered from the Century series. The 28 stood for the approximate engine power, which in this case was 2,750 hp. Actually, a 624 model was initially planned, but this was replaced by the 628 due to the engine power demanded by the market. Of the 628, 135 were made for US customers, 46 for Mexican customers and five went to Australia.

Although the Century 628 was built until December 1968, the 630 model with 3,000 hp was offered in parallel from 1965. This model was optionally available with a high hood in front of the driver's cab and was built 77 times for US customers up to 1967. The almost identical M-630 was manufactured in the Montreal Locomotive Works, of which 55 were delivered to customers in Canada and 20 went to Mexico. Of the 26 engines received by the Montreal Locomotive Works, eight had the broader “Canadian” nose. The M-630 had already received some technical features from its successor, the M-636.

The 636 with an output of 3,600 hp was offered from 1967 and differed from its predecessors in the different arrangement of the fan grilles in the intercooler area. It had special Hi-Ad bogies that could better put the power on the rails. Since the demonstration locomotive repeatedly had to struggle with technical difficulties, the number of orders was limited. Only 34 examples were built for US customers and 29 more were built under license by AE Goodwin in Australia.

It wasn't until 1969 that the Montreal Locomotive Works began manufacturing the M-636, which was based on the original 636. It had some teething problems fixed and used the revised 251F series engine instead of the 251E. By 1980, a total of 185 units had been sold and also went to Mexico and Australia. The one-off M-640 was tried out, which had an 18-cylinder engine with 4,000 hp and three-phase traction motors.