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.
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.
