No carbon credits, yet, from these engines. This primarily reduces Nitrous Oxides. All of the newest engines are at about 80-90% less NOx the biggest health "criteria" pollutant, that form acids in fog, and leads to Ozone-Smog formation. It has paid to do it because of the fuel savings and more reliability with the new systems, although they were told do it or face very stiff regulations, including forced electrification.

Until they were forced to clean up the junk they were putting into the air, nobobdy in the railroad industry paid much attention to the problem. Engines were pretty filthy until the mid 90's. Today new engines are very clean, more reliable, and much more fuel efficient by comparison.

Many newer units and gensets also reduce some green house carbons-CO2 by better fuel economy, though. But the catalysts, and newer engine pollution control systems, are primarily for NOx reductions. The acid formed from the NOx from engines, combined with fog, back in the "good old days", caused us to have to cover our automobiles at night at Taylor Roundhouse to keep the paint finish from being eaten off the metal, while you could actually feel the acid burn your skin.