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Starting in 1997, the EPA (Environmental Protection Agency) passed regulations on exhaust emissions for diesel locomotives. Five tiers (Tier 0 through Tier 2) were identified, with each higher tier having a requirement for cleaner emissions. Tier 0 applied to locomotives built between 1973 and 1992. Tier 1 applied to locomotives built between 1993 and 2004. Tier 2, which was the first real major decrease in emissions, went in to effect for locomotives built in and after 2005. In 2008, the EPA added Tier 3 and Tier 4 requirements. Tier 3 applied to units built between 2012 and 2014, with Tier 4 starting in 2015. The Tier 4 standard required an 89% reduction in nitrogen oxides and a 93% reduction in hydrocarbons compared to Tier 0 standards. Compared to Tier 3 standards, nitrogen oxides required a 74% reduction and hydrocarbon required a 77% reduction It took both GE and EMD years to develop locomotives capable of meeting the regulations. The designed were complicated and the units were expensive. In the case of EMD, they could no longer use a two-stroke engine, and instead had to move to a four-stroke (12-cylinder) engine, similar to GE. Now, in 2023, UP still only has 87 SD70ACe-T4s (the Tier 4 compliant unit from EMD) on the roster. Most railroads have instead chosen to rebuild older units as rebuilds have lower emissions requirements and are far cheaper. One of the 87 SD70ACe-T4s is seen here, third unit at Carr on this southbound manifest.
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