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. 

The DL-103 to DL-110 were the first diesel locomotives built by ALCO. The DL only stood for “Diesel Locomotive” and the individual models hardly differed from each other. The odd numbers stood for normal locomotives with cabs (so-called A-units), while the even numbers denoted the cab-less B-units or boosters. As with most American diesel locomotives of the time, the carbody was designed as a self-supporting construction with a streamlined front. The shape of the front with the three-piece windshield was designed by the German-American industrial designer Otto Kohler. All locomotives had an output of 2,000 hp, which came from two six-cylinder inline prime movers with turbochargers. Although four traction motors were sufficient to transmit the power, three-axle bogies, each with a carrying axle in the middle, were used, as in the EMD E series.

A total of 74 A units and four B units were built between 1939 and 1945. By far the largest proportion of these were the 60 A units ordered by the New York, New Haven & Hartford Railroad. All other customers received a maximum of four units, which were often used in front of prestigious passenger trains. The Second World War had made procurement difficult, since the production of purely passenger locomotives was prohibited and locomotives that were equally suitable for freight and passenger trains were also difficult to obtain. Finally, the New Haven only used their locomotives to haul passenger trains during the day, while hauling freight trains at night. Between 1949 and 1951 they were modernized and, among other things, the side windows in the engine room were removed, which were only used for decorative purposes. Despite this, the locomotives' service life was not very long, as they were soon replaced by more modern locomotives. Some were still visually matched to their EMD competitors, but most fell victim to the scrap press in the 1950s.

To change secondary lines in New South Wales from steam to diesel, AE Goodwin built the DL531 under ALCO license. With six axles, this had a weight of only 74 tons and a six-cylinder with 1,050 hp. Until 1970, the NSWGR received a total of 165 locomotives in standard gauge, which it designated as class 48. Otherwise it was also known as RSD8.

The South Australian Railways also bought 45 DL531 in three different gauges and designated them as class 830. The Silverton Tramway bought three, and customers were also found in Brazil, Peru and Pakistan. In Australia, many were retired from the state railways in the nineties and sold to private railways, many of which are still in use today.

The HH series were the first diesel shunting locomotives that ALCO mass-produced in large numbers. They were also among the first land vehicles with a diesel-electric transmission. “HH” stood for “High Hood” while the number stood for the engine power in hp. The first HH600 demonstrator was sold to New Haven after testing in 1931. In 1934, industrial designer Otto Kuhler revised the shape of the locomotive body, giving it more rounded corners.

The first variant had a McIntosh & Seymour 531 with six cylinders. It wasn't until 1937 that the HH900 followed with a turbocharged variant of this engine. In 1938 the HH660 followed with the slightly more powerful naturally aspirated engine of the 538 series and from 1939 finally the HH1000 with the turbocharged variant of the engine of the 538 series. A total of 177 locomotives were built, of which the HH600 was the most successful with 79 units due to its long production period. The individual railway companies operated a maximum of twelve of each model.

With the development of the new Type 244 engine, ALCO introduced the PA passenger locomotive in addition to the FA freight locomotive in 1946. Not only did the propulsion electrics come from General Electric, the streamlined and modern design of the car body also came from Ray Patten, head of GE's design department. The PA got the more powerful sixteen-cylinder engine and stood on two bogies with the A1A wheel arrangement. What all PA and FA have in common is that black smoke came out when accelerating due to the lack of air in the turbo lag.

With a 2,000 hp engine, 169 PA1 and 39 PB1 without a cab were built. From 1950 onwards, 81 PA2 and eight PB2 were built with 2,250 hp. Three PA-2s went to Brazil with a gauge of 5 ft 3 in. Some railroads also used their PA and PB in freight traffic, but the number of units remained behind EMD's E series due to lower reliability. The Massachusetts Bay Transit Authority used its locomotives until the late 1970s and then sold them to the Ferrocarriles Nacionales de México, which continued to use them until 1981.

In 1940, Rock Island approached ALCO and ordered a new type of diesel locomotive that would be equally suitable for switching and road service. Instead of the cab units that were widespread at the time, a locomotive with a load-bearing frame and narrower hoods was developed, also known as the “hood unit”. The resulting design was characterized by the fact that there was an unobstructed field of vision when running in both directions. This meant that it could be used without restrictions for switching, but also in road service without any cutbacks. It was usually running with the long hood forward, even if this meant a poorer view forward. Thus the new breed of road switchers was created, which was soon adopted by all North American manufacturers.

A total of 466 of the RS-1 were completed at ALCO and three more at the Montreal Locomotive Works by 1960. This is the longest production run of any diesel locomotive for the North American market to date, and also meant that they continued in production for several years after their successors were discontinued. The first locomotives, which were actually intended for Rock Island, were delivered to the US Army due to the USA entering the war.

Although a steam generator was available as an option, the RS-1 was rarely used in front of passenger trains. In freight service, with its 1,000 hp, it was primarily used on branch lines. The large six-cylinder ALCO 539T proved to be very durable, with many locomotives still being used in the 21st century. The RSD-1 and RSC-1 were developed as six-axle developments for lighter rails, with the latter only four axles being driven.

The RS-2 was built between 1946 and 1950, with 368 being built at ALCO and nine in Montreal. It differed from the RS-1 mainly in the new 244 series V12 engine with 1,500 hp and a slightly more rounded body. The new prime mover did not prove to be as reliable in use as the old one. 64 were delivered to Mexico and 46 to Brazil. The locomotives for Brazil had a gauge of 1,600 mm.

The RS-3 achieved the highest number, of which 1,272 were built between 1950 and 1956, with 146 coming from Montreal. It was created at a time when Fairbanks-Morse and Baldwin were also bringing their road switchers from 1,500 to 1,600 hp to keep up with competitor EMD. 98 went to Canadian customers and 46 to Brazil. Here, too, a six-axle variant with four powered axles was developed as the RSC-3.

To overcome the reliability problems of the 244 series prime mover, 98 RS-3s were rebuilt between 1972 and 1978 with EMD 567 engines. The engines, including the fan system, came from retired EMD E8. These were named RS-3m to distinguish them. A few of these are still in use in 2022, although some RS-3 that have not been rebuilt have survived into the 21st century.