As I recall, the issue with EGR on the 710 was not that it didn't work, it actually worked very well; but the EPA tier IV particulate levels were much lower when published than anticipated, and EGR helps NOx but not particulates, leading to a need for impractically sized exhaust filtration.
A quote from a
Trains magazine article ('Lenz' is the head of EMD's engine development):
"Using a pair of borrowed Union Pacific SD70ACes fitted with large roof boxes for emissions testing, Lenz says EMD successfully got the 710 engine up to Tier 4 requirements. But while the tests were successful, the resulting design was too heavy, too cumbersome and not cost competitive enough to be marketable. In particular, the fuel economy was worse than for Tier 3."
As far as the US and European diesel loco market is concerned, there endeth 75+ years of EMD 2-stroke diesel development...
My understanding is that the main reason EMD and GE didn't go down the SCR route to reduce NOx emissions is because their railroad customers did not want to have to supply Urea (for the SCR system) as another consumable at every fuelling point. That's quite a big issue when locos wander round all over the continent (US, Canada and Mexico) - 'run through' operation of freight motive power is very common these days.
Most passenger locos tend to stay in a small area (or use a very small number of fuelling points) so using SCR/Urea for those isn't a big issue, hence the Cat C175 in the the EMD F125 and the
Cummins QSK95 in the
Siemens 'Charger'.
