Streetsblog Massachusetts editor Christian MilNeil has just asked a very delicate question on Twitter about battery power for public transportation. In-motion charging (IMC) is a positive technological development for buses, wiring part of a route in order to provide electric coverage to a much broader area. So why not use it for trains? The context is that the government of Massachusetts is doing everything in its power to avoid wiring commuter rail; its latest excuse is that a partly-wired system with battery-electric trains is cheaper. So how come IMC works for buses but not trains?
The answer is that trains and buses differ in ways that make fully wiring a train much more advantageous for equipment cost while costing less compared with IMC-style partial wiring – and the size of trains makes the equipment cost much more prominent.
The cost of a single-deck electric multiple unit (EMU) other than high-speed rail is about $100,000 per linear meter of length, and appears to have changed little over the last 10-20 years. I have a list of recent tramways built in Europe for that cost, a shorter one of subways (including more outliers due to procurement problems or bespoke designs), and some standard citations for commuter rail EMUs. For the latter, here is a recent example of a Coradia Continental order in Germany: 200M€ for 32 trainsets, 20 with five 18-meter cars and 12 with four, or 75,000€ per linear meter.
In contrast, battery-EMUs (BEMUs) are far more expensive. Comparing like with like, here is a recent Coradia Continental BEMU order for Leipzig-Chemnitz, which line should have long been wired: 100M€ for 11 three-car, 56-meter long trainsets, or 160,000€ per linear meter.
Buses do not display such a premium. Trolleybus advocate Martin Wright writes a comparison of battery-electric and trolleybuses for Vancouver, and suggests that equipment costs are largely the same in the North American market (which is expensive by European standards). TU Berlin’s Dominic Jefferies and Dietmar Göhlich find that the base cost of an electric 12-meter bus is 450,000€, rising to 600,000€ with battery (p. 25); this is a premium, but it’s small, almost an order of magnitude less than that for trains per unit of length. Kiepe says that the cost of rebuilding 16 12-meter trolleybuses with IMC for Solingen is in the single-digit millions.
How come trains display such a large premium for batteries over electric traction supplied by trackside distribution (catenary wire or third rail) and buses don’t? This is not about the cost of the batteries: Jeffries-Göhlich cite a cost of 500-800€/kWh for a battery pack on a bus, and while Alstom hasn’t said what the battery capacity of the Coradia is in kWh, based on the range (120 km) and this slide deck about BEMUs (or PDF-p. 22 of a VDE study about EMUs and BEMUs), the capacity is likely around 700 kWh for the entire three-car train, with a cost about an order of magnitude less than the observed cost premium over EMUs.
Rather, the issue is likely about fitting the batteries on the train. Railvolution reports that to fit the batteries, Alstom had to demotorize one of the three powered bogies, reducing the maximum power drawn from 2.16 MW to 1.44. As a byproduct, this also somewhat hurts performance, increasing the stop penalty from the train’s maximum speed of 160 km/h by 15-20 seconds (46 empty or 51 full for an EMU, 60 and 71 respectively for a BEMU).
The cost of wiring
The cost of trolleybus wiring, at least judging by industry brochures such as that of UITP, is linear in route-km. This makes IMC attractive in that it cuts said cost by a factor of 2 to 3 on a single route, or even more on a route that branches out of a common trunk. For this reason, IMC is ideally suited for branched bus networks such as that of Boston, and is less valuable on grids where it’s uncommon for multiple bus routes to run together for a significant portion, such as the systems in Chicago, Toronto, and Vancouver.
But rail electrification does not quite work this way. Overall, the cost of wiring is mostly proportional to route-length, but the cost appears to be split evenly between the wire and the substations. A full-size commuter train in a major metropolitan area like Boston would be drawing around 7 MW while accelerating; a Citaro bus has a 220 kW diesel engine, or 125 in the electric version. Even taking into account that buses are slower and more frequent than trains and thus run at much higher frequency per route-km, there’s nearly a full order of magnitude between the substation costs per km for the two modes.
The upshot is that while IMC saves the cost of installing wire, it does not save a single penny on the cost of installing substations. The substations still need to fully charge a train in motion – and derating the train’s power as Alstom did does not even help much, it just means that the same amount of energy is applied over a longer period while accelerating but then still needs to be recharged on the wire.
How benefits of electrification scale
Electrification has a number of benefits over diesel power:
- No local air pollution
- Much less noise, and none while idling
- Higher reliability
- Higher performance
- Much lower lifecycle costs
The first three are shared between externally-supplied electric and battery-electric power, at least when there’s IMC (pure battery power is unreliable in cold weather). The fourth is a mix: BEMUs have better performance than DMUs but worse than EMUs – whereas with buses this flips, as trolleybuses have performance constraints at trolleywire junctions. The fifth is entirely an EMU benefit, because of the high cost of BEMU acquisition.
The first two benefits are also much more prominent for buses than for trains. Buses run on streets; the pollution affects nearby pedestrians and residents as well as waiting riders, and the idling noise is a nuisance at every intersection and whenever there’s car traffic. Bus depots are an air quality hazard, leading to much environmental justice activism about why they’re located where they are. Trains are more separated from the public except when people wait for them.
In contrast, the last benefit, concerning lifecycle costs, is more prominent on trains. The benefits of electrification scale with the extent of service; that the acquisition cost of EMUs is around half that of BEMUs, and the lifecycle cost is around half that of DMUs, means that the return on investment on electrification can be modeled as a linear function of the fleet size in maximum service.
A US-standard 25 meter railcar costs $2.5 million at global EMU prices (which the US was recently able to achieve, though not anymore), and twice that at BEMU prices. 40-year depreciation and 4% interest are $162,500/year; a single train per hour, per car, is around $3,000/km (this assumes 50-60 km/h average speed counting turnaround time), or $6,000 counting both directions, and lifecycle maintenance costs appear to be similar to initial acquisition cost, for a total of around $12,000/km. At $2.5 million/km, this means electrification has an ROI of 0.5% per peak car per hour; a single 8-car train per hour is already enough for 4% ROI.
The numbers don’t work out this way for buses. Workhorse city buses run every 5 minutes at rush hour, and may occasionally run articulated buses, but the capacity is still only equivalent to a single hourly train; in the absence of IMC, electrification of buses is therefore hard to justify without the additional environmental benefits. But those environmental benefits can be provided at much lower cost with IMC.
The upshot of the above discussion is that the reasons to electrify buses and trains are not the same. Bus electrification benefits center environmental and environmental justice: diesel buses are noisy and polluting and have poor ride quality. The only reason to wire buses at all rather than go for unwired battery-electric buses (BEBs) is that BEBs are not reliable in freezing temperatures and cost far more than diesels due to their downtime for charging.
But rail electrification is different. The environmental benefits are real, but less important. Train depots have not been major sources of air pollution since the steam era, unlike bus depots. The primary reasons are technical: equipment acquisition costs, maintenance costs, performance, reliability. And those overall advantage EMUs over BEMUs with IMC.