That's true, so you're technically correct about it happening during Bush 43, but the process started in 1994 with the court case that required EPA to set standards, based on the requirements set forth in the act, which EPA stalled, at first. Part of the Soto vs. EPA was over EPA's failure, which was brought about because of the clean air act, that 41 signed. the It started with Tier II, then Tier 3, then Tier 4 standards, to achieve the requirements of emission reduction targets consistent with the available technology.

I sat on a technical review committee that funded Tier 4 demonstration projects and locomotive research prior to 2004, using a State funding source that required reductions in excess of current standards, that put together a number of Tier IV demonstration implementations.

Without the Clean Air Act amendments signed by Bush 41, we'd still be looking at 35-50 grams of NOx per brake horse power hour, of the 1980's locomotive fleet. As opposed to a mostly Tier III locomotive fleet today, that is emitting about five grams of NOx in the same operational mode. This is also what pushed heavy duty CNG with trucks, since it requires less hassle, and the fuel availability has increased, and cost have been more stable, for large based fleets, like buses, trash trucks, UPS, and other delivery fleets, all of which have been "historic" on-road mobile sources of diesel related air pollution.

Bush 43 simply let the law his father had signed take its course, and 43's EPA was even criticized for delays in setting these tougher standards at the time. Although, I also thought, at the time, and have been proven correct since, that this was a premature standard for locomotives, as there are still significant cost and operational issues, that have not, as yet, been "efficiently" solved.

I had, back then, suggested that emissions calculations include measures of efficiency (weighted by ton mile??? per unit of emissions emitted, which would have (could have?) leveled the playing field a bit, for RR's versus trucks. That idea, of course, flew like a lead balloon with the enviros, while I was still doing this kind of work...needless to say.... EVIL DIRTY DEVIOUS DIESEL! They knew all about it, from growing up with Thomas....