They had a spare trainset in Washington, they broke it up and used the cars to make the other trainsets longer.
Wait! When did that happen?
Last I knew they had 5 trainsets and that's how many were originally built, although one set was supposed to work the LA to Las Vegas corridor and was named the Las Vegas. WADOT owned the Mt Baker & Mt. Rainier, while Amtrak owned the Mt. Hood & Mt. Olympus. WADOT then brought the Las Vegas and renamed it to the Mt. Adams. But all trains started with 12 cars and at least as of 2010 still had 12 cars. And I don't recall seeing a longer train last October when many of us rode the Cascades during the Gathering.
AFAIK they still have 5 trainsets and operate 5 trainsets.