The EPA Tier system does nothing to reduce carbon emissions, and a Tier 4 diesel is thermodynamically pretty much just as energy-inefficient as a Tier 1 diesel. Once carbon emissions are priced properly to reflect their effect on our future habitat, the business case for electric transportation infrastructure will make a lot more sense financially, not just on the basis of performance. (Those armadas of 737s and A320s will be priced out of the skies too.)

As for operating cost, the FTA database has good data that shows commuter rail O&M costs are pretty much the same regardless of diesel or electric. The LIRR, NJT, Metra and Caltrain are all in the same ballpark and electrification of Caltrain won’t change that much. Assistant conductors cost more than prime mover maintenance.