EPA Tier 0

This category is for all locomotive built between 1973 and 2001. Any unit within this time frame are EPA Tier 0 compliant.

http://www.epa.gov/nonroad/locomotv/420f09025.pdf page two of this PDF explains, how much output there are of emissions. This also gives you an idea of how much more locomotives have become eco-friendly.

g/bhp/hr stands for grams (of output) per horse-power-hour.