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. 

During the Second World War, the Soviet Union ordered high-traction diesel locomotives from the USA via the Lend-Lease Act. These should be used primarily in the southern parts of the country, where only little water was available for the operation of steam locomotives. In response, ALCO equipped the RS-1 with two additional powered axles. Despite the same engine power, a higher starting tractive effort was now available, but a decreasing power with increasing speed.

70 RSD-1s were sent to the Soviet Union via the Trans-Iranian Railway, with 12 engines already falling victim to submarines at sea. There they were given the designation Д^A20 (D^A20), which stood for “Diesel, ALCO, 20 tonnes axle load”. They served there as a basis for the ТЭ1 and its successors. Another 87 locomotives went to the US Army and six to the Ferrocarriles Nacionales de México.

36 RSD-4 were built in 1951 and 1952, based on the RS-3. They had the new 244 series engine with 1,600 hp and were all delivered to US customers. Since the generator was considered too weak, the RSD-5 was built with a new generator from 1952. A total of 167 RSD-5s were built for US customers and 37 for Mexico.

Since the gas turbine locomotives built for the Union Pacific in the 1950s did not manage to establish themselves as standard locomotives for very heavy trains, the development of particularly powerful diesel locomotives was tackled again in the 1960s. However, the power of the individual prime movers was limited, so it was decided to combine the power plants of two locomotives each on a common frame. Thus, there were two 16-cylinder series 567A, each with its own generator, on a frame, which in turn stood on two four-axle bogies. 

The first model was the DD35, which was built 30 times in 1963 and 1964, mainly for the Union Pacific. Their power plants were identical to those of a GP35, giving a combined output of 5,000 hp. They did not have a driver's cab and were intended for use as a B unit together with classic diesel locomotives. Since the four-axle bogies at the front of the train greatly reduced the service life of the rails, no A-units with a driver's cab were initially built. It was mostly used with two DD35s between two GP35s, giving a total output of 15,000 hp. It was not until 1965 that 15 DD35A cab-units followed, built exclusively for the Union Pacific. Both types had in common that the sandboxes were initially in the engine room and sand flying around caused problems in the electrical system there, which led to the sandboxes being relocated to the circulations.

The high point of the development of the eight-axle EMD locomotives were the DDA40X “Centennial”, of which 47 units were built between 1969 and 1971 for the Union Pacific. New 16-cylinder series 645 prime movers were now used, which brought the total output to 6,600 hp. Thanks to further developments on the bogies, it was no longer a problem if such a locomotive led the train. Inside, many new assemblies were used, which later came into other EMD locomotives. The driver's cab was of the wide design used on the FP45.

In day-to-day operation, however, it soon became apparent that such a powerful locomotive hardly brought any operational advantages compared to two smaller ones and was less flexible to use. For this reason, no further eight-axle large diesel locomotives were later developed, apart from special narrow-gauge machines with low axle loads. All later models had only four or six axles and were or are mostly used at least double-headed. Even today, when the current standard diesel locomotives have an output of more than 4,000 hp, trains with ten locomotives are formed on mountain routes. Also the already built DD35s and DD35As were phased out in the early 1980s and the DDA40Xs in the middle of the decade.

The Brazilian Estrada de Ferro Vitória a Minas required more powerful diesel locomotives than the previously used EMD G16U at the end of the 1960s. So far, the ore trains with up to 150 cars have been pulled by two or four G16Us and there are plans to pull them with just one or two single-engined locomotives in the future. At the time, EMD's most powerful single-engined diesel locomotive available was the SD45 with the 3,600 hp 20-cylinder. However, it turned out to be problematic for use with the EFVM that due to the meter gauge used, the traction motors were smaller and therefore not powerful enough to implement the full engine power with a total of six traction motors. 

The solution was to use four-axle bogies similar to those used on the DD35 or DDA40X, so that each traction motor only had to provide an eighth of the total power. This resulted in the DDM45, of which a total of 83 were delivered to the EFVM from 1970 onwards. The type designation was made up of the wheel arrangement DD, M for a different track gauge and the SD45 as the basic model. The locomotives could each replace two of their predecessors and were more economical to use. They usually drove double-headed with 160 ore wagons with a total weight of 12,000 tons or triple-headed with 240 wagons and 18,000 tons. In terms of performance, they were surpassed from 1991 by the newly arrived General Electric Dash 8 with 4,000 hp. Shortly before, two examples had been equipped with more modern Caterpillar four-stroke engines, but due to the high costs of the conversion, no further ones were converted and the two were later dismantled. Some engines came to the private FCA after the turn of the millennium, but were returned to the EFVM in 2008.

The next power level after the GP30 was the GP35, that was introduced in 1963. Thanks to improvements to the traction motors, 2,500 hp could now be obtained from the EMD 567D prime mover, which has been in use since the GP20 and has not yet been able to exploit its full power potential in four-axle locomotives. Production took place again in both the USA and Canada and a total of 1,334 pieces were made. Of these, 24 went to the Canadian Pacific, two to the Canadian National, 55 to the Mexican State Railway, two to the Sonora-Baja California Railway and the rest to US customers. The largest customers in the US were the Santa Fe, the Pennsylvania Railroad and Southern Pacific, which each ordered more than 100 examples. Many railroads got their GP35s while trading old F-series and Alco locos to EMD. There, the bogies were reused together with the traction motors and fitted with new superstructures.

Nine months after the GP35, the GP28 was added to the range. It was a cheaper variant that only had an installed power of 1,800 hp. The power came from the same engine, which only had a Roots supercharger instead of the turbocharger and was therefore in roughly the same power class as the GP9 engine. However, the GP9 was still using the old EMD 567C, while the GP28 had the more modern EMD 567D1. However, sales were limited and only 31 were built, 15 of them for Mexico and Peru. Other very similar locomotives were created when the Burlington Northern bought some GP9s from the Northern Pacific and brought them up to the same technical level. These locomotives are referred to as GP28M.

At the end of the 1950s, General Electric was still new to the diesel-electric road switcher market and was trying to win market share from market leader EMD. When GE introduced the U25B, EMD felt compelled to develop a comparable locomotive. The U25B delivered 2,500 hp, while EMD's four-axle models delivered a maximum of 2,000 hp. In addition, the GE locomotive had a sophisticated system for supplying fresh air for cooling various assemblies, which at the same time kept dirt and dust away, but still made maintenance easy. Therefore the GP30 was developed while the GP20 had just appeared on the market. The type designations, which indicate the power in hp x 100, had only recently been introduced, they now deviated from them again.

The problem with fresh air was solved by redesigning the airflow, which gave the GP30 a distinctive crest. This ran from above the driver's cab to the fan housing of the optionally available dynamic brake. With the GP20, EMD encountered the problem of being able to convert a maximum of 2,000 hp with four traction motors. With minor adjustments, it was finally possible to get another 250 hp out. Although not quite at the level of the U25B, it was hoped that railroads that had been using EMD locos for a decade or two would buy this locomotive. This recipe worked because, despite the shorter production time, 948 units could be sold, while the competing model was only built 478 times. Union Pacific was the only company to buy 40 B units, some of which were equipped with steam generators for use in front of passenger trains. Over time, many GP30s have been rebuilt to extend their life. Some were still in use after 50 years and some have just been rebuilt, which will further extend their service life.

With the successors to the GP28 and GP35, the more powerful variant with 3,000 hp was again offered as the GP40. The major innovation of this generation was the introduction of the new EMD 645 engine, which had 645 cubic inches or 10.6 liters per cylinder. To accommodate the new engine, the locomotive's frame and hoods were lengthened by three feet. These locomotives could also be recognized visually by the three fans of the same size on the rear part of the hood. Between 1965 and 1971, in the USA and Canada a total of 1,187 units were built for US customers, 16 units for the Canadian National and 18 units for two Mexican railway companies. The Norfolk & Western Railway was the only customer to make use of the option of having a driver's desk installed in each direction of travel. Some engines were built as GP40P and GP40TC for operation in front of passenger trains. These were longer to accommodate the train heating.

The GP38 was offered as a less powerful variant from 1966, which achieved an output of 2,000 hp without a turbocharger. It was also an economic success with 706 units, six of which were delivered to Mexican customers. Derived from the GP38 was the GP38AC, which had an AC generator. In the years 1970 and 1971, a total of 21 units went to the Canadian Pacific and 240 to US customers.

In the meantime, the GP39 had emerged, which was powered by a smaller engine with twelve cylinders and a turbocharger. It produced 2,300 hp and was therefore slightly above the GP38. Only 23 of them were built, and the Burlington Northern later had some GP30s and GP35s converted to GP39s.

EMD offered revised variants of the GP38, GP39 and GP40 in the “Dash-2” series of road switchers from 1972 onwards. Basically nothing changed in the power train, so that the GP38-2 had a V16 without turbocharger with 2,000 hp, the GP39-2 a V12 with 2,300 hp and the GP40-2 a V16 with 3,000 hp. The main innovation was a modular electronic equipment that allowed easier maintenance and originated from the DDA40X. Improvements to the power train concerned efficiency and reliability.

The GP38-2 was the most successful model with a total of 2,213 units sold. Of these, 1,799 remained in the United States, where the Missouri Pacific, Penn Central and Southern each bought more than 200 units. 156 machines were exported to Mexico and one to Saudi Arabia. 257 were built in Canada by GMD and delivered to Canadian customers. 51 of these were built for the Canadian National with the wider “comfort cab”. The latter are referred to as the GP38-2W by railway fans to distinguish them, although the manufacturer never used this designation. Thanks to its simplified design, the engine achieves such a high degree of reliability that in 2020 most GP38-2s were still in use.

The GP40-2 was also available from 1972 and, with a total of 1,143 units, produced a slightly smaller number than the GP40. The largest customer in the USA was Baltimore and Ohio, which bought 218 units. Three GP40P-2 were built for the Southern Pacific, which were intended for use in front of passenger trains. 44 locomotives were exported to Mexico. 279 were built in Canada, all of which stayed in the country. By far the largest part of this was the Canadian National with 268. Over the years many aging GP40-2 have had their turbochargers removed to improve reliability and henceforth continued to exist as GP38-2.

While the GP39, with its poorer reliability compared to the GP38, did not become an economic success, EMD was able to sell larger numbers of the improved GP39-2. From a total of 239 produced locomotives, the ATSF took 106 pieces, which were completed by 40 others from the Burlington Northern after the merger to BNSF. Also included in the GP39-2 fleet are some rebuilt GP40-2s that have been given electric train heating. Since a significant part of the engine power is no longer available for traction, they were designated as GP39H-2.