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.

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. 

After ALCO, Baldwin and Fairbanks-Morse already had successful road switchers on offer, EMD did not offer the first model in this category until 1949 with the GP7. The “GP” stood for “General Purpose”, which was intended to indicate the wide range of possible uses and was often reinterpreted as “Geep” in everyday language, based on the Jeep brand off-road vehicles. It was based on the F7, but no longer had a self-supporting body and instead hoods which rested on a supporting frame. The power came from the same sixteen-cylinder type EMD 567B with 567 cubic inches per cylinder. The combustion air was only supplied via a Roots supercharger, which initially provided an output of 1,500 hp. 

The GP7 could already be ordered with a wide range of options, for example the orientation of the driver's desk could be chosen and a steam boiler for heating passenger trains could be accommodated in the short front hood. The short hood could also be ordered in a lower version for better visibility. There was also a B-Unit without a driver's cab on offer, but only five of these were ordered by the Santa Fe. The remaining 2,729 machines built up to 1954 all had a driver's cab. Of these, 112 were built by General Motors Diesel in Canada and mostly delivered to Canadian customers.

The direct derivative was the GP9, which was technically based on the F9 and was built between 1954 and 1963 a total of 4,257 times. Of these, 165 were built as B units. Outside of the USA and Canada, a couple of locos were also sold to Mexico, Venzuela and Peru. The power of the GP9 was 1,750 hp, which was achieved by increasing the number of revolutions, while other measures made maintenance easier. The successor to the GP9 was the GP18 from 1959, which achieved an output of 1,800 hp. Of the 405 manufactured vehicles, 40 went to Mexico and 15 to other countries outside North America.

Jersey Central Lines GP7 No. 1523 in 2010 at Tuckahoe, New Jersey

Around the same time as the GP18 came the GP20, which had the same turbocharged engine as the SD24 introduced in 1958. This engine was the first to use the mechanically assisted turbocharger that EMD still uses today. This had a clutch with which it could be connected to the engine and thus became a combination of exhaust turbocharger and mechanical supercharger. At low engine speeds, when the exhaust turbocharger was not yet able to build up sufficient boost pressure, it only ran as a supercharger. At higher engine speeds, the clutch was opened, allowing the exhaust gas turbine to do the charging on its own. The reduced power compared to the SD24 was due to the fact that the DC traction motors available at the time were limited to a maximum of 500 hp and therefore a maximum total power of 2,000 hp could be implemented with a four-axle locomotive. Since only a meager 200 hp power increase was available compared to the GP18 and there was still a significantly higher maintenance effort, the GP20 was not that popular and was therefore only built 260 times. Some machines were even later equipped with engines of the 645 series, which only had a Roots supercharger and, thanks to the larger displacement, achieved the same power.

After the market launch of the GP7, customers expressed the desire for a six-axle variant with a lower axle load. The GP7 was equipped with two three-axle bogies while maintaining the same engine power and the frame was lengthened, resulting in the SD7. The abbreviation stood for “Special Duty”, but is now interpreted as “Standard Duty” due to the widespread use of the SD series. Despite the lower axle load and better traction, many customers shy away from the purchase due to the higher acquisition and maintenance costs, which is why only 188 units were sold between 1951 and 1953.

Similar to the GP9, the SD9 was introduced in 1954, which also had 1,750 hp. 515 examples were built until 1959, 44 of which went abroad. Foreign customers were Korail from South Korea and mining companies from Chile and Venezuela. Shortly after the GP18, the SD18 was created, which in turn had the same prime mover. It was sold 114 times, and this time more than half was exported. 45 pieces went to Brazil, while Korail bought another 15 pieces.

Even before the SD18, the SD24 was offered from 1958, which was the first EMD locomotive to use the mechanically assisted turbocharger. Since the six traction motors used could handle up to 400 hp, the output of the prime mover could be set to 2,400 hp. Although this did not fully utilize the engine's potential, full power could be accessed even on the highest passes in the Rocky Mountains. 179 SD24s were built, all going to US customers. In addition to its 30 SD24, the Union Pacific ordered 45 B-Units SD24B without a cab.

Under the impact of the past World War II, the ongoing Korean War and the smoldering Cold War, the US Army Transportation Corps saw the need for a diesel locomotive that could be used almost anywhere in Eurasia in the event of war. Thus in 1952 the order for 13 such locomotives each went to EMD and General Electric. The requirements included a 1,600 hp loco that had a smaller loading gauge generally usable in Europe and an axle load that was lower than the typical axle loads of American locos. In addition, the gauge of the bogies was to be changeable. Although it was not intended to be used on narrow-gauge routes, the Soviet gauge (4 ft 27/32 in), the Iberian gauge (5 ft 5 21/32 in) and the Indian gauge (5 ft 6 in) were to be covered in addition to the standard gauge (8 ft 8 1/2 in).

Like its competitor, the EMD locomotive was given the designation MRS-1, which stood for “Military Road Switcher 1”. The special requirements and the changeable track drove the costs far up, which reached almost half a million dollars per locomotive. Since EMD lost the competition, the 13 sample pieces remained. Two locomotives were taken to Manitoba and Alaska to test diesel locomotive operation in arctic conditions. One example made it to Europe, being used extensively in West Germany, France and Belgium for training, testing and promotional purposes. In Belgium, this led to the purchase of the EMD locomotives built under license by NOHAB. The remaining locos spent most of their lives in storage in preparation for war. When the Pentagon's new doctrine focused more on moving material by air and made the diesel locomotives obsolete, they were removed from storage in 1970 and allocated to military sites.

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.

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.

As with the first generation, the GP28 and GP35 were once again offered as six-axle variants with the same engine power. The SD35 with 2,500 hp was offered from June 1964 and 360 were built for customers in the USA. 100 of these went to the Southern Railway, followed by 80 for the Norfolk & Western. From July 1965 the SD28 with 1,800 hp was also offered. Since a six-axle locomotive of this power class was apparently not very attractive on the market, only two were sold to the Columbus and Greenville Railway and four to the Reserve Mining Company.

Just one month after the launch of the SD35, the Seaboard Air Line ordered 20 passenger locos to replace their old E-series locos and still be able to be used as freight locos if required. They were based on the SD35 with a steam generator and gear ratio for 95 mph and were designated the SDP35. A total of 15 more were ordered, ten of them by the Union Pacific. When Amtrak took over passenger service, the SDP35s remained with their operators and were used in freight service and shunting. The four pieces of the Louisville & Nashville were even reconfigured before delivery and delivered without a steam generator. A passenger variant of the SD28 also appeared, six of which had been ordered by the South Korean state railway. These were significantly lighter and shorter than the other six-axle locomotives.