From "The Electro-Motive Story" - last paragraph (bolding added by me):
In 1962, EMD had produced its 25,000th locomotive, making it the worlds leading manufacturer of Diesel locomotives. In 1965, the second-generation 40 Series locomotive was introduced. The new 645 Series Diesel engines powered it and in 1972 EMD celebrated its 50th anniversary, commemorating Electro-Motive Engineering’s 1922 date of incorporation. The next generation 50 Series locomotive was introduced in 1979. By 1983 EMD had built 50,000 locomotives and 1984 saw the introduction of the 60 Series locomotives with the 710 Series engine. It had the newly developed wheel adhesion control system. Burlington Northern placed the largest ever single order with EMD in 1993 when it ordered 350 SD70MAC units equipped with the new AC traction technology. 1997 marked EMD’s 75 years of excellence and 1998 saw the addition of the 265H engine to join the 710
Series already being built.
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From my limited experience as an Engineer, I would suggest this:
1st generation: E and F units and geeps and SD's thru GP20
1.5 generation: EMD GP30 & GP35 and all GE "U" series.
2nd generation: all with 645 engine, mostly GP40, SD40, 1966-1972
2.5 generation: the EMD -2 line and early GE "C" series
3rd generation: The EMD 60 series, beginning with the 710 engine, and GE's dash 7 series.
3.5 generation: Early EMD 70 series and GE's dash 8 series
4th generation: EMD SD70ACE, SD70M-2, GE dash 9 and EVO models
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Just my thoughts. Having more operating experience with 3 SD45's and 1 SD40-2, I was surprised at the differences between the earlier electrical systems and the -2 electricals.
One could argue that the GE "U" series were true 2nd generation, but they sure didn't survive long when compared with EMD's 40 series. I will let others speculate on where the SD80 and SD90MAC43 and SD90MAC60 along with the 6000hp GE's belong.