Late, but here's the relevant info for MAX:
According to TriMet Code, Chapter 28, D(2), blah blah, no weapons, blah blah..
...except in accordance with administrative rules as may be promulgated by the General Manager or otherwise provided by law. Where possession of such weapons cannot be prohibited by law, a person in possession of a weapon may not display or carry the weapon in a manner which is likely to result in fear or alarm by other persons or District employees.
The wording is questionable, since the whole "public menace" and "disturbing the peace" are popular means for the PoPo (popular nickname for the Portland Police, for those outside Portland,) to 'get' people carrying openly. But based on that, carrying openly will likely produce "fear or alarm" here in Portland. All it takes is one inspector to get scared, and you're kicked off.
Now, if you're willing to put up with the PoPo coming and dragging you away, and being excluded - then later fight it in court, more power to you.
Note: Open carry is *LEGAL* in a few cities TriMet serves, such as Hillsboro and Gresham. That means without a CHL, you can open carry a loaded firearm at the two ends of the main MAX line; but not anywhere in between. Also, because of the goofiness of the Oregon Constitution and laws, cities can only ban open carry of *LOADED* firearms. (Which includes removable magazines, unfortunately - but does *NOT* include revolver speedloaders.) So if you have your ammunition separate from your firearm or its magazines, you can carry open, even in downtown Portland, even without a CHL. But that is still asking for a world of abuse from the police.
I remember a few years ago someone was walking through downtown Portland with a shotgun. The police (who were apparently significantly more civil at the time - hard to believe for a life-long Portlandian,) stopped him, interviewed him quickly, ensured his shotgun was unloaded, and carried on. (They *DID* stay nearby, I'm sure partly to keep an eye on him, but also partly to reassure scared people that what he was doing was legal.)