I've seen plenty of Cummins tier 3 equipped loco rebuilds, as well as other engine upgrades and replacements, to things like heavy construction equipment, and based on a lot testing and use that I'm familiar with, they seem to work just fine..... The tier 4 engines are undergoing similar monitoring and testing, and we'll see how that does.
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Maybe, my experience, in actually having had to breath some of the pure crap from those old smoking Baldwin, Alco, GE, and even EMD diesels for many years, makes me somewhat less nostalgic for the days of filthy air, than some?

We used to have to cover our cars at night, at Taylor, to keep the NOX that combined with the fog to create nitric acid from eating the paint, and we were breathing that crap.