We've gone too far in attempting to make things clean to the point where we are actually causing more pollution than we are saving. Both urea and non-urea truck systems are prone to failures, and when they fail they emit many more times the pollution than the engine would have if it had been left alone and remained at 2000 technology. A study by the
Atlantic Air Pollution Technology Task Force stated that over the life of the so called "clean diesel" trucks, total pollution is greater than a 2000 built truck due to increased fuel consumption, hazardous materials created by catalist systems, emissions created during typical failures of emissions equipment, urea spillage and other factors.
It was also noted in the study the increased pollution created by the "premature" scrapping of older locomotives that normally would have been remanufactured, but in order to comply with emission standards, old locomotives were scrapped (adding to the waste stream and consuming resources during scrapping and handling---recycling has costs too) and new locomotives built to replace them. Noted further, railroads were forced to pass on the additonal costs of buying new vs. remanufacturing old locomotives (and emissions equipment adds approx. $730,000 to the cost of a new locomotive) on to shippers which resulted in more shipments going by the less fuel efficient trucks.
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A report by Clean Air America noted a conflict of interest in the EPA emission standards board in that it is dominated by former GE employees and the resulting standards tend to
favor both GE locomotives and jet engines. (In case you wondered how GE locomotives keep
meeting emission standards and still put out black turo-lag smoke, this may be why.)