One of the details in Mike Sanders' rapid_rail_presentation is the proposal to use diesel multiple units (DMUs) — a passenger rail vehicle propelled by an onboard diesel engine. This is unique because most commuter rail operators run conventional locomotives pulling (or pushing) conventional passenger rail coaches. Regional examples of conventional commuter rail are Chicago's Metra, Dallas' TRE, Minneapolis' Northstar, and Nashville's Music City Star).
DMUs are easily confused with electrically-powered light rail vehicles and modern streetcars, and the difference is slight: other than the powertrain and the lack of overhead wires, DMUs that run on freight rail tracks must conform to strict crash regulations. This, unfortunately, makes them heavy. At the same time a DMU can be (arguably) cheaper to operate on routes with light demand.
To make matters even more confusing, one of the few places in America where DMUs operate — New Jersey Transit's River Line — is actually called a light rail line. We can't even tell you that the terms "commuter rail" and "light rail" are even 100% distinct, since systems bearing either label can perform similar goals — transporting commuters to and from the urban core — over similar distances. A good rule of thumb, however, is that light rail better serves urban environments with closer stops; commuter rail better serves suburbs with stops spaced further apart… regardless of the vehicle type or fuel source.
We had a chance to ride a DMU transit route on a recent trip to Portland. The TriMet's Westside Express Service has been in operation since 2009 and serves four suburbs. We'd like to be the one to tell you that this route was trouble-free to construct and operate, but that would be a lie.
Regardless, the day we rode WES it was glitch-free, on-time, full of passengers, and included in our $4.75 all-day transit pass (unusual for US commuter rail). The sensation was a mash-up of riding any other train with the subtle reminder that a large diesel engine was underfoot (and releasing particulate pollution, although not nearly as much as if all riders had chosen to drive congested I-5 instead). Bikes were onboard — you can't really avoid them in Portland, even if you tried — and the easy transfer from light rail, plush seats, and a friendly conductor made our brief trip a pleasant experience.
As for the Sanders proposal, the reliance on DMUs for all-day service on multiple lines would indeed make it unique in all of North America, perhaps the world. The company that built the WES vehicles has reincorporated in Ohio — with a Missouri-based partner, no less — and plans to resume production soon. Hopefully they will engineer improvements that make the vehicles more reliable for daily service.
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