My guess is you have one of the 300 HO Master Unit cars built by Fairfield Traction Models for Northwest Short Line. (I'm guessing NWSL brought this out due to the Portland connection; they're usual offerings were Pacific Northwest steam, especially logging.)
The Master Unit was an important step towards the "standard" street car, which culminated with the launch of the Presidents' Conference Committee (PCC) car in 1936. The Master Unit was a standard set of components, but only within the Brill group of companies, which included Osgood-Bradley, whose version was the Electromobile. Operators had some flexibility in car length and other details (there was even a single-truck prototype unit), and it could be built in single- and double-end versions. Almost all were city cars, save for the 10 previously noted Red Arrow suburban/interurban cars.
In terms of history, there's a good history of J.G. Brill published a few years ago, which is also available on Google Books. Also, in addition to the cars mentioned above, the Pennsylvania Trolley Museum (in Arden, PA, south of Pittsburgh) has one of the Red Arrow Master Unit cars, on display and in operation, and also has a Red Arrow Brillliner and PCC car.
If you visit, Arden, be sure to check out the single-end and double-end Pittsburgh "Jones" cars, an earlier, operator-level standardized car.