Railvolution reports FTA numbers that say the average CO2 emissions of the New York City Subway are 0.17 pounds per passenger-mile (48 grams per passenger-km). That’s the equivalent of 114.6 passenger-mpg of gas, if you prefer to think in those terms. The presentation gives average seat occupancies, which we can also confirm with the NTD; it works out to about 4 car-mpg of gas. Other agencies can have somewhat different numbers, based on train efficiency and especially the local sources of power generation, e.g. BART has very low emissions coming entirely from the fact that the Bay Area has ample hydro power resources.
New York’s emission number, 4 mpg, may be familiar to you as roughly the emission-efficiency of regional diesel trains. Per ton of car mass the regional diesel trains do slightly better, since the regional train in question weighs 40 tons vs. 33-39 for New York’s subway cars, but this comes from making fewer stops. At agencies with very dirty power generation, such as the Chicago L, and even ones without very dirty power, such as the energy-hungry Washington Metro, the numbers are even lower, even though they’re electric and the regional diesel trains are not.
What we see is then that railroad electrification does not add too much to fuel economy. The question is then why the situation for cars is so different. The Nissan Leaf’s EPA-rated fuel economy equivalent rating is 99 mpg – almost as good as the New York City Subway, better than nearly all subway systems in the US. But if we try to break it down based on energy consumption, we get other numbers; the EPA just massaged the numbers to make plug-in hybrids look good.
The Leaf’s energy efficiency is 0.34 kWh per vehicle-mile, pardon the mixed units; the FTA’s numbers for major US subways range from 0.186 kWh per passenger-mile in high-seat-occupancy New York to 0.388 in low-seat-occupancy Chicago. This is not 99 mpg, unless one uses a fairly clean mixture of fuels; with the New York mixture, it’s 63 vehicle-mpg. So right off the bat, the official numbers underestimate the Leaf’s CO2 emissions by 36%, and overestimate its CO2 efficiency by 57%.
But even that doesn’t take care of inefficiencies in generation. Well-to-wheels, plug-in electric cars have about the same emissions as regular hybrids. This confirms the rough numbers we’ve seen from trains. The Tesla Roadster, a very fuel-efficient car, gets even better energy-efficiency even wells-to-wheels, but it also has much lower electricity consumption, and to get the right numbers it assumes electricity is generated from natural gas rather than coal.
Bear in mind, all of this assumes certain things about the grid mix. At the current US grid mix, on average electrification does not impact carbon emissions. Of course, since people need electricity for reasons other than transportation, any regime in which carbon emissions fall is one in which electricity becomes lower-carbon, and this would tilt the field in favor of all-electric vehicles, both cars and trains.
So, why electrify, if there’s no carbon emission benefit, why electrify? Two answers: air pollution, and, for trains, performance. Electric trains outperform diesel ones, and also cost less to operate in terms of both energy and maintenance. But electrification should be sold only on grounds that are in fact correct.