I just wanted to add that the WES RR line is an ACTIVE freight railroad corridor. There's sufficient freight traffic making time separation schemes for running non FRA compliant DMU or light rail trains impossible. Others have pointed out that the RR line is owned by a private RR corporation, not by the public transit agency.
It's true TriMet could have bought Bombardier BiLevel cars and diesel locomotives. I'm not so sure I agree with the statement that several bridges aren't strong enough to support these cars. Freight cars weigh much more and run down this line every day. But operational costs of using DMUs with 300-350 HP diesels will be significantly cheaper than 3000-3500 HP diesel locomotives. Getting 2 mpg vs 2 gpm, ie, 4 times better fuel milage. The Colorado Railcar DMU will also be much quieter than a diesel locomotive.
Bombardier's cars would be a better choice if TriMet thought ridership would grow faster in the future. The TRE in Dallas-Fort Worth ridership is up to over 11,000 riders per day, using 4 Bombardier cars behind the locomotive during rush hours. The diesel locomotive can pull 10 Bombardier cars, at which point it reaches maximum fuel efficency.
But TriMet is planning on using 1 DMU car initially, adding a second trailer car on the train with the highest ridership. It's also a startup line, the first commuter rail in Oregon. Additionally, the WES train leaves the rail corridor in Beaverton to reach the transit center. Using smaller, more efficient equipment makes sense, at least initially, with the much lower ridership and street running.
Other, relatively low ridership commuter trains are being planned today around the United States. I believe the Stadler GTW DMU train design can be modified to meet FRA standards cheaply. It's just a manner of designing and building stronger passenger sections of their three part trains. But, there's got to be sufficient demand before they will do this.
Stadler Non-Compliant DMU trains presently used in America:
NJT RiverLine
Austin's CapMetroRail
I believe Dallas' Dart, Fort Worth's T, Smart, and even Amtrak may order enough FRA compliant DMU trains in the near future in sufficent numbers to make this chore profitable for Stadler. Let's hope so, because I can forsee many more orders for this DMU over the Colorado Railcar, whose delivery on time and on budget record must be the worse in the history of train manufacturing.