Electrification and Carbon Emissions

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.


  1. ant6n

    If you pick the Leaf, which is a very modern vehicle in its class, you should probably also consider more lightweight trains with regenerative breaking. I don’t know what the numbers are for the state of the art, but I found the numbers for the Berlin S-Bahn. I know their most recent rolling stock from the 90’s uses regnerative breaking. Apparently they used ~0.12 KWh/(P*Km) in 2006 (before the meltdown), according to their environmental report, page 14, 2nd figure.

    Of course you’re making a point that’s more about order of magnitudes, and you’re right of course.

    • Alon Levy

      JR East claims to have an emission efficiency of 12 grams per passenger-km, i.e. four times as much as the New York City Subway. Part of it is due to higher occupancy and lighter cars, but I think the numbers also ignore the energy conversion efficiency, just like the EPA’s PHEV numbers.

    • Danny

      Regenerative braking advocates tend to overstate their energy savings. The thing is, yes they do put energy back into the grid, but there is a problem with assuming that every watt placed back in the grid is a watt saved. The truth is that energy pushed back into the grid usually is absorbed by transmission lines, surge protectors, and transformers and released in the form of heat. People don’t rush to turn on their microwaves whenever a train stops. With a very well distributed grid with a large transit system consistently and predictably making stops, you might actually be able to use 50% of the energy that is regenerated.

      There is potential for very large energy savings from regenerative braking, but those savings will come when supercapacitors have the capacity to store energy on the train, as opposed to pushing it somewhere else.

      • Alon Levy

        The other issue is the hybrid cars already have regenerative braking; electric cars don’t have an advantage over hybrids there. In principle, diesel-electric trains are also hybrids, but they don’t have regenerative braking – and at any rate the Regioshuttle is diesel-mechanical.

      • Beta Magellan

        Chicago’s per-passenger emission don’t come so much from rolling stock but (in addition to lots of coal-derived electricity) service characteristics—although the north side Red Line, Brown Line and Blue Line between O’Hare and the Medical District are all well-patronized, other parts of the line are underutilized (Green and Yellow, both of which are getting infill stations but could definitely support more), are well-patronized but have low ridership densities (Dan Ryan Red, Orange), or compete with one another (Red and Green on the south side, Pink, Blue and Green on the west—in fact, in the late eighties/early nineties serious consideration was given simply demolishing the Lake Street El and replacing it with an extra Metra stop in Oak Park). I’m not sure if this is as much of a factor, but Chicago’s services alsohave a very heavy downtown peak—the only lines with any appreciable reverse commute are the Red Line (where many transfer to Purple to get to Evanston) and the Blue Line (Rosemont/O’Hare), so they’re often running a lot of nearly-empty revenue trains during peak hours.

        • Nathanael

          Chicago could use some method of storing trains downtown (so that they could run fewer, shorter trains off-peak), but they don’t have any.

          • Alon Levy

            It’s probably cheaper to run empty reverse-peak trains than to come up with land for this in the Loop. Unlike cars, trains are expected to pay their own way for parking.

          • Nathanael

            Yeah. Makes you wonder whether building over some of the old railyards was wise, though.

  2. orulz

    Another reason to electrify in the US? Natural gas and coal come mostly from domestic sources. Diesel fuel comes from the middle east.

    Nuff said.

    • Alon Levy

      Natural gas, sure. But burning coal when there’s an alternative is basically an act of war against people in coastal floodplains. If the US wants to start a war with Nigeria and Bangladesh, I’d rather it were done in Congress and with a national discussion of why those countries are a threat to America, rather than by defaulting to the energy policy that most benefits coal magnates.

      • Eric

        It will be much cheaper to build dikes around the coastal regions of Nigeria and Bangladesh, or to pay for some of the residents to relocate, than for the world to stop burning coal.

        • Beta Magellan

          This—giant seawalls are easy to build, easier to maintain, have zero negative effects on coastal ecosystems, keep the oceans’ pH in check, and actually scrub mercury from the water and particulates from your lungs.

          • Eric

            I didn’t say no negative effects. I said “much cheaper”, i.e. a lower level of negative effects. With the trillions of dollars saved by not crippling our fossil-fuel-based economy, you could do quite a bit to remedy all sorts of environmental problems, including some of the ones you mention.

          • Beta Magellan

            How do we keep the oceans’ pH from dropping further without curtailing CO2 emissions? Or do you believe our continued use of fossil fuels make us rich enough to sequester CO2 faster than we can pump more out.

          • Eric

            The oceans’ pH will drop and we will have to deal with it. Or how many billion dollars is each coral reef worth to you?

          • Beta Magellan

            The best round number I could find was around $30 billion a year (pdf) (including existing coastal protection benefits), although that estimate was from 2007 and comes from a news article in Science, not an actual letter—most of what I have on-hand deals with narrower subjects. I don’t believe the $30 billion figure includes non-reef based effects either (such as decreased shellfish output). There’s better literature about US output and acidification, but that isn’t a good substitute for global impact since the US isn’t normally considered all that reef-dependent—typically you’ll read that the main economic brunt of this would be born by places like southeast Asia. A big complication could involve corals in cold, deep waters, whose response to acidification is not fully understood. And measurement even in shallow, warm-water reefs remains a problem—most less-developed countries either don’t monitor their reefs very well or haven’t started monitoring acidification yet.

            Given the general direction climate science has moved in the past few years (more pessimistic), I’d expect a higher number today.

          • orulz

            There are currently no reactors in the US that can generate power from the thorium fuel cycle, and the likelyhood of building a bunch of new nuclear reactors in the US took a big hit after Fukushima, especially given that federal subsidies / credits will be needed to make them economically viable.

            Agreed, however, that the US and close allies have a lot of uranium, though.

            The reference to Borat was meant to be a joke but I guess it was off the mark. I realize that the US and Kazakhstan share many strategic interests with regards to the middle east.

        • Beta Magellan

          We get along just fine with Kazakhstan (arguably too fine—there’s a probably solid argument that we could could have been more upfront about Nazarbayev’s increasing authoritarianism, though most of my knowledge of the place is geographical/anthropological so I’m not really the best person to make that argument).

          You’re right about never mentioning Borat, though.

      • Nathanael

        Even better, sunlight is supplied free locally.

        And no, solar panels do not require any uncommon or exotic materials.

          • Nathanael

            Well, I happen to know that panel efficiencies are going to go up by a factor of ten. The details are confidential.

            Heating? Well, yeah, electricity remains inefficient for heating, despite heat pumps. But if our biggest problem is heating, the way to approach that is through insulation first (the Northeast has, for the most part, appallingly badly insulated building stock). Solar thermal heating is quite effective even in the winter in the Northeast, apparently. After that you can see how much natural gas and propane use is left.

          • Alon Levy

            Well, for new buildings, it’s not much of a problem. I hear that Danish houses don’t even have artificial heating – they use passive solar design. Danish winters are about as cold as New York winters, so in principle it’s possible all along the NEC; doing the same in Upstate New York and the Midwest may be harder.

            The issue is how difficult it is to retrofit old buildings. What I’ve read on the subject is conflicting; do you know if it’s easy to insulate your average Victorian-era brownstone?

          • Andre Lot

            That is not true, Alon. Many buildings in Northern Europe use passive insulation design. I live in a level-2 of such houses.

            While it slashes the energy loss, it still requires decent and powerful heating systems to be in place, because there are limits to what insulation can achieve. Completely passive buildings do exist, but they are an oddity: if a cold spell + heavy wind condition happens, they become frigid inside, like 12-15 oC inside and no way to heat it but bring electric radiators on, which is woefully inefficient.

          • Adirondacker12800

            do you know if it’s easy to insulate your average Victorian-era brownstone?

            Insulating existing buildings is labor intensive which makes it expensive. The materials are relatively cheap. It’s very easy if your labor consists of writing the check to the contractor.

          • Richard Mlynarik

            Existing structure retrofitting = Labour intensive + low material cost + large energy savings
            ought to have been the perfect equation in a world with far too many humans and with hideously high per capital consumption.

            But no.

  3. ant6n

    These sort of comparisons show that the most energy efficient form of non-active transportation in most parts of North America is car sharing.

  4. Valkraider

    It is hard to compare these vehicles of completely different use-cases. Regional diesel trains (typically and ideally) make very few stops. Transit oriented trains make more stops but still very few stops on average, and all the stops are pre-determined (not counting abnormalities such as accidents, breakage, or other unplanned stops). Also, all trains have no need for steering – they run on rails.

    Automobiles however operate in very different use-cases. Which is not only why the difference in efficiency crops up – but also why we need to keep differing power sources around for autos for the forseeable future.

    Diesel automobiles are very efficient for long runs at highway speeds – but they are not very efficient in stop & go urban traffic. Electric vehicles are very efficient in urban stop & go environments – but really lose their efficiency at any kind of highway speeds. The Leaf is extremely efficient around town – even other hybrids can’t really compete (yet). The instant torque of an electric powertrain lends itself well to city use. Add to that the ability to *completely* shut off when at a stop – with no penalties (such as having to restart an ICE during stop+start functions).

    Highway use, however, does not need the surge in torque, and has no need for stop+start capability – electric cars are not as good at cruising long periods of time at high speeds.

    The secret to a good future with electric cars – assuming that the car and battery technology improves the way most things do through the years – is that with electric power sources we can generate the fuel in many many ways, via a distributed network.

    Imagine a future with a very decentralized electric production infrastructure. Not only can we keep improving power-plants efficiency and cleanliness, but we can find new and exciting ways to generate electricity. Small vertical wind turbines on every house in areas that are breezy. Solar panels on every roof. Small fuel cell stacks in the basement of homes and businesses, generating electricity from a variety of fuels. Vibration power generation near roads or intersections. All kinds of possibilities, most renewable, all clean and domestic – and if distributed enough they should be resilient to natural disasters and terrorism.

    That is an advantage that gasoline and diesel will never have – regardless of how good ICE engines get, we still have to get the fossil fuels gasoline and diesel from somewhere. Most homes cannot put in a refinery (even a biofuel one) in their back yard – but there are a variety of electricity generation capabilities that a regular homeowner can employ – and they get better and more affordable every year.

    • Nathanael

      This is the key: electricity generation can be *made* renewable fairly easily. And thanks to the trend in solar prices, it will only get easier.

      Diesel generation? Can’t. Biodiesel is an expensive process by comparison, though if it’s done with waste oil, that’s different.

    • Beta Magellan

      There’s also the issue of maintaining the electrification infrastructure—although I’m not sure what the cutoff is (anyone here know of a study?), at a certain point it overwhelms decreased vehicular maintenance costs (and new ridership from improved performance).

    • Alon Levy

      Where do you see this pushback? I can think of a few places in historic city centers, for which there’s catenary-free tram operation, and in NIMBY-prone suburbs that think they’re historic city centers. I’ve heard of very few cases of this in mainline service (LOSSAN, for one), though.

  5. The Overhead Wire (@theoverheadwire)

    Most of it is not coming from railroad lines but rather light rail and streetcars. Everyone seems to want the wireless operations, specifically in cities. That’s a question I get asked fairly often. I know that it was a problem in Austin because the main parade route is congress avenue, I’ve heard mumbles about it in Charlotte and other places as well.

    • Eric

      Is it possible to have overhead wires in and around the station (where a disproportionate amount of time and acceleration occurs) and rely on (smaller) batteries in between stations?

      • Beta Magellan

        The only systemI know of that does something like that is the ULTra personal rapid transit line at Heathrow, where vehicles run under their own power along guideways (obviating a third rail) but do quick recharges at stations. This comes with the obvious caveat that the ULTra vehicles are small; I’d also guess that the pods form queues at stations, which might provide more time for recharging. It’s also worth noting that a lot of the currently-proposed streetcar circulators have very short stop spacing, so only having wires around stations still means a lot of wires.

        There’s also the British Rail Class 139, which uses flywheel storage and a diesel engine, though it has low passenger capacity and is currently only being used to connect a town at the end of a .8-mile branch to a Birmingham-bound commuter rail line.

  6. R. W. Rynerson

    There are a lot of peripheral issues with electrification that contribute to generating ridership, particularly in regard to noise and cleanliness of the vehicle. I can remember noticing that middle-aged women would walk to the rear of an Edmonton trolley coach and continue their conversations, while that would never happen on a Diesel bus. The flip side is that numerous arbitrary service reductions over the years left that system with too shallow an operation to warrant the literal overhead costs. Electric rail operations can go into environments such as tunnels and street operations that can get them closer to customers, while Diesels deal with complicated ventilation and noise issues. Issues such as these are lost in straight mechanical engineering comparisons.

    Straight comparisons between fuel efficiencies or pollution produced between any type of auto and public transit are also fallacious, because customers’ behavior changes if they are a public transportation user. When I come home on the (Diesel) bus or the (electric) Light Rail, I get off at the grocery store and then walk home. The parking lot at Whole Foods is full of cool cars, including hybrids, that are as or more efficient as the bus I rode, depending on load factors, except that many of the motorists do not practice trip-chaining, but instead run errands one at a time, curtailing actual energy efficiency. Some of them are idling their engines to keep the AC or the heater going. And, they’re the customers who may drive to the bigger store when they can’t find what they want right now, resulting in the need for more parking there.

    Mechanical comparisons are mainly of value when similar types of service are included in the comparison.

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