I too would like the answer to this. I'll be arriving at 11pm next week and have a hotel downtown. I read on here that a cab is about $22 to downtown from the airport. If it really takes that long to get that train at night, we will definitely take a cab.

I know the train takes about 30 mins. But when I took it I got right on after my flight, and am not sure how often they run. I've never paid anything to use the train.

The Red Line runs every 15 minutes everyday beginning at 5 AM, with the last train leaving PDX at 11:30 PM. The trip from PDX to downtown Portland costs $1.55 and takes just 38 minutes.
If you don't pay, and a fare inspector catches you, you will pay a hefty fine.

Schedules are different on weekends, but on weekdays it looks like the MAX train leaves PDX every 15 minutes or so until 10:30pm, then every 25-30 minutes until midnight. The ride to downtown is about 30 minutes, and will drop you within a few blocks of the Hilton (get off at Pioneer Square).

Schedules are not real clear, but take a look at www.tri-met.org. You should be looking for the light-rail (MAX) schedule to Portland and/or Hillsboro.

I know the train takes about 30 mins. But when I took it I got right on after my flight, and am not sure how often they run. I've never paid anything to use the train.