For 70 years, in a period where transit and railroads underwent massive change, one thing has remained constant: the propulsion of choice for any new American commuter rail line is diesel. But that was before $125 a barrel oil. If we build commuter rail in Houston, it’s worth asking how it ought to be powered.

The diesel locomotive has a lot going for it. Diesel is easily portable and packs a lot of power per pound. The external infrastructure is simple; trains can even be fueled directly from a tank truck. Diesel locomotives are off-the-shelf, they share most of their components with common freight locomotives, and it’s easy to find people who know how to maintain them.

But diesel pollutes, it produces greenhouse gases, and it’s gotten a lot more expensive. And there are alternatives.

The gold standard for railroad propulsion is electricity, as it has been since it was invented. Electric locomotives are quieter, more powerful, longer-lasting, lower-maintenance, and more energy-efficient than diesel. They emit no pollution from the vehicle itself, and if the electricity is provided from hydroelectricity, wind, solar, or nuclear, they do not pollute at all. In passenger use, electric trains lend themselves to powering every axle, which increases acceleration and reduces trip times. And the electric motors can be used to brake the train as well as accelerate it, returning energy to the system, saving 15% or more.

Electric propulsion has only two downsides. One is minor: the visual appearance of the wires. Modern systems are very simple, and it’s worth noting that the extremely appearance-conscious Swiss have no objection to errecting them in their treasured mountain landscapes. The second is more significant: perhaps $6 million a mile to install the wires. That immediate cost is what taxpayers and shareholders see. But on a heavily travelled route, the lower operating costs –and better service — can justify that. The world’s busiest railroad systems — Germany, France, Britain, Italy, Japan, China, India, Russia — all have significant electric routes. In the United States, New York, Philadelphia, D.C./Baltimore, and Chicago have electric commuter rail, and San Francisco is preparing to electrify its line.

No other alternative to a standard diesel locomotive is as common as electricity. But there are other viable technologies. Hybrid locomotives are in use for freight trains, and they could easily be adapted to passenger use. Likewise, natural-gas powered locomotives are a proven technology. Both are still dependent on fossil fuels. But another proven alternative is not: in 1955 the West German railways introduced ETA 150 battery railcars, which could carry 86 passengers 180 miles at up to 60 mph on one battery charge. 232 were built, and they remained in everyday service for 40 years. Could 2008 technology do better?

In 1998, with oil at $20 a barrel, diesel propulsion might be an obvious choice for commuter rail. In 2008, with $125 a barrel, it seems foolish not to consider other options. Whatever the choice is, it will stick for while: commuter locomotives last for 20 years or more, and electric infrastructure is cheapest when it’s done at the same time as track upgrades.

Above, the worst of both worlds: MBTA’s Boston-Providence route is one of two U.S. commuter rail lines that runs entirely under electric wires but uses diesel locomotives.