Responding to some posts above, I found another article that has some specifics. And reading more, it does sound like they are looking at actual emissions data in making their decisions.

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August 30, 2017
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The first project, a partnership with Omni TRAX, Inc., will replace one pre-model year 1973 non-regulated and unmodified 1963 built EMD SW1200 diesel Switch locomotive with a new Tier 4 Knoxville Locomotive Works (KLW) SE10B T4L four-axle diesel Switcher. The replacement locomotive will be a single-engine EPA Tier 4 certified and ARB verified Switcher locomotive. The new locomotive is more than 90 percent cleaner than the existing, high-polluting locomotive it is replacing. Based on past usage records, the existing switcher’s annual fuel usage is 16,000 gallons/year. The existing switcher is used within the Stockton Terminal and Eastern Rail Yard in Stockton and throughout 10 miles of track, eight miles of which are through residential neighborhoods within the city of Stockton and two miles through commercial and residential zoned areas. The Valley Air District is funding slightly less than 50 percent of the total cost.

The second project, a partnership with the Metropolitan Stevedore Company, will replace one non-regulated and unmodified 1984 built GE C30-7 six axle diesel line-haul locomotive with a new Tier 4 Knoxville Locomotive Works (KLW) SE24C DET4L six-axle diesel line-haul locomotive. The replacement locomotive will be a single-engine EPA Tier 4 certified and ARB verified line haul locomotive. The new locomotive is more than 90 percent cleaner than the existing, high-polluting locomotive being replaced. Based on past usage records, the existing locomotive annual fuel usage is 26,673 gallon/year. The existing GE C30-7 locomotive is used to breakdown incoming rail car trains and for unloading and exporting shipments of mineral commodities. This locomotive operates within the Port of Stockton property. The Valley Air District is funding approximately 30 percent of the total cost.

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http://escalontimes.com/section/11/article/6733/