DMU train in San Diego. Image by mrpeachum licensed under Creative Commons.

This article was first published on April 9, 2013. We're running a version of it again because we still lack DMU trains.

In the Washington region we have Metro and commuter rail trains, with light rail, streetcars, and Bus Rapid Transit (BRT) all in the works—and of course, regular buses. But one common mode we don’t have is DMU trains, which bridge the gap between light rail and commuter rail.

DMU stands for Diesel Multiple Unit. DMU trains are ideal for operating on routes that look like commuter rail, but at almost light rail frequency. They go over long distances, with infrequent stations, usually on or adjacent to freight tracks. But instead of usually coming only at rush hour, trains typically come all day long, as often as every 15-20 minutes.

That’s a great service model for suburban corridors that need something better than rush-hour MARC or VRE service, but are too far away for light rail and don’t have the density to justify the costs of Metrorail.

DMUs, and their electric cousin EMUs, are used in Philadelphia, New Jersey, Portland, San Diego, Dallas, and Austin. They’re proposed in even more cities.

One big advantage of DMUs over traditional commuter trains is that DMUs can operate on-street, like light rail. That makes integrating them with downtown areas much easier, because it frees DMUs to go anywhere, rather than only to a city’s main rail hub.

Austin DMU on-street. Image by Paul Kimo McGregor licensed under Creative Commons.

All MARC and VRE trains to DC must go to Union Station, because all the long distance tracks through DC go to Union Station. Not only does that constrain route planning, it’s also a limit on capacity, because there are only so many platforms at Union Station. But a DMU could go anywhere.

There are not currently any plans for DMU lines in the DC region, but there could be. DMU would be a great solution for Maryland’s proposed Charles County corridor or Fairfax’s Route 28. Years ago officials considered light rail for those corridors, but being far out in the suburbs and with infrequent stops, they wouldn't generate enough ridership to justify the costs. DMU might be more appropriate.

In the long term it might also make sense to convert some of MARC and VRE’s existing lines to DMU, or to supplement them with more DMU trains. That would give them more operational flexibility, and could increase the how often MARC and VRE could support running trains. But MARC and VRE are established as traditional commuter rail; adding new completely different equipment would be complicated and costly, and could actually decrease capacity at peak times.

MARC and VRE also have to use tracks owned by freight companies. DMUs can be used in mixed company with freight, although that requires federal approval. But if the freight lines are already using their tracks to capacity, which is common in the DC area, then there’s no room for more trains no matter what they look like.

DMU isn’t Metro, and it isn’t light rail. DMU trains can’t do all the things those modes can do. It’s not an appropriate mode where frequent stops are necessary. But for long corridors with infrequent stops and moderate capacity needs, it’s often a good choice. The fact that we don't have any in our region today doesn't mean it might not be the best choice in the future.

Cross-posted at BeyondDC.

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Dan Malouff is a transportation planner for Arlington and an adjunct professor at George Washington University. He has a degree in urban planning from the University of Colorado and lives in Trinidad, DC. He runs BeyondDC and contributes to the Washington Post. Dan blogs to express personal views, and does not take part in GGWash's political endorsement decisions.