I've been involved in three different efforts to look at the possible electrification of RR's in the past 30 years to reduce locomotive emissions. It is not cheap, somewhere between 4 and 75 million per mile (depending on a very wide variety of "assumptions" and highly varied construction conditions). If you are looking for the "low hanging" fruit, when looking at large scale and cost effective carbon dioxide emissions, I would not be looking at RR electrification as anything even close to the most "cost effective" investment to do it.

If you truly want the biggest worldwide reduction in CO1 emissions, try replacing Chinese and Indian coal (some of the dirtiest low btu/high CO2 coal that is burned) in power plants with "clean" Monterey Basin "California" fracking gas, would reduce those emissions by over 2/3's. Replacing every air conditioner compressor over 30 years old, using tax credits to individuals and grants to things like schools, improving heat and ac loss by improved windows and better insulation, replacing all "thermal" light bulbs with LED's, replacing every TV set older than 15 years old would all reduce vastly more CO2 at a far lower cost per "ton" (some of these with real and significant user cost "savings") than the tens of billions that would be necessary to electrify Americsn RR's.