Not really any good MTB fun accessible by public transit in this town, unfortunately.
Forest park has a good long worn out road (Leif Erickson) with a couple of teaser fire roads coming off it. There are some good single track trails there but the good stuff there is all illegal.
You can do a couple laps around Powell Butte. But it's kinda boring, IMO.
If you're into dirt jumping there are jumps to hit by Rocky Butte.
Probably the most feasible "fun" single track action via transit is to be had in Scappoose. Take the CC Rider (Columbia County Rider) out to Scappoose. Get on your bike and backtrack south on HWY 30 a couple miles, take a right on Rocky Point Road (it's kinda a Right then Left off the hwy.) Then ride 1.1 miles up to the first blue gate on the right. Nice trails out there. Make sure you don't miss the last CC Rider back to the big city.