The core environmental benefit isn't from the fuel-switching of the train, its from the population switch in transportation mode. Electrification would mean more reliable, rapid, frequent, (and hopefully more connected) transit, which causes an increase in ridership. Especially with a connected N-S station the benefit is in the regional alteration of the housing dynamics as well, and the increased ability to live along transit lines to reach employment centers. The reduced need for long-distance, traffic clogged, SOV travel is an amazing public benefit. Personally, I'm dying to see what the GLX will do to the travel patterns of Somerville residents. Then extract that change to the greater region.
Side-note... I love that you're taking the energy-grid perspective, but looking at a snapshot of the current electrical grid is mildly disingenuous - if the electrification of the transit system is looking ahead 20 years, then look ahead in the future of the power grid too. The Quebec Hydro line will already be complete, the offshore wind market will be thriving, and battery technology will eliminate most peaking plants. If there is any lesson to be learned, look at how quickly technologies can take over. Gas power exploded over the last 20 years, and renewables paired with customer demand changes will be just as impactful. Quick presentation of the ISONE grid interconnection queue if energy is anybody's thing...
https://www.iso-ne.com/static-asset..._mcbride_presentation_april_10_2019_final.pdf