Submitted by Breck, who says:

Here’s a neat little animated map and explanation that show how the new MAX Orange Line will tie into the rest of the system in downtown Portland. Like many other transit fans, I too have wondered how the Orange Line will work and why it won’t just be a simple extension of the Yellow Line. It is all explained here, and the reasoning seems to make pretty good sense.

Transit Maps says:

Like Breck, I’ve also wondered how the Orange Line will tie in to the downtown part of the MAX system, and this neat little GIF confirms what I’ve suspected for a while: most of the time, the Orange Line will simply act as an extension of the Yellow Line. The reason it isn’t officially Yellow is two-fold: flexibility of operations (interlining with the Green Line or turning back to Milwaukie at Union Station if required); and… marketing. Basically, it’s easier for TriMet to promote a “new” line to the public (exciting!) rather than an extension of an existing one (yawn!). As the TrIMet blog itself says:

We think the Orange Line deserves its own recognition as a pioneering endeavor. Besides showcasing the first bridge of its kind in the U.S., the Orange Line features a host of sustainable elements like eco-roofs, eco-tracks and bioswales to capture stormwater runoff. And it serves a distinct corridor stretching from the region’s urban core to growing communities, setting it apart as our region’s newest light rail line.

While i’m still not sure if I agree with the decision to not make the whole thing Yellow and be done with it, at least the changeovers between Yellow and Orange as shown in the GIF make decent sense. Coming from the north (Expo Center), trains are Yellow until they reach Union Station. Presumably at this point, riders from the north will mostly disembark at the downtown stations (still believing they’re on a Yellow Line train), while new riders from Union Station onwards will see incoming trains as Orange Line trains to PSU and Milwaukie. Similarly, Orange Line riders from Milwaulkie will mostly get off in the city centre, and any new riders will instead be greeted by a Yellow Line train to Expo Center.

Source: TriMet’s “How We Roll” blog – link no longer active