There is very little public transportation to common trailheads, but here is some:

• Central Oregon Breeze Goes between Portland and Bend. They will make stops at Government Camp if you pre-arrange.
• Sandy Area Metro Goes from Portland Gresham Max station to Timberline Lodge. Stop at Zigzag and you could walk over to the West Zigzag Trail or up Hunchback Mountain. Stop at Mt Hood Village and hike over to the Wildwood Recreation Site and up the Boulder Ridge Trail into the Salmon-Huckleberry Wilderness.
• Columbia Gorge Express New for 2016. May 27 to Sept 25. Shuttle from Gateway Transit Center to Rooster Rock and Multnomah Falls.
• Portland International Airport ground transportation A list of shuttles and other that will transport you to/from Cascade Locks, Timberline Lodge, and other locations.
• Skamania County 2015 weekend schedule from Vancouver to North Bonneville, Stevenson, Carson, and Dog Mountain
• pcta linkpcta announced that skamania county has more runs on weekends and they stop at Washington Columbia gorge trailheads. Paid for with grants to improve access to federal lands.
• Eugene Airport Ground Transportation Several possibilities.
• Lane County Transit District Transit goes from Eugene as far East as McKenzie Bridge which is about 21 miles from the PCT and the Three Sisters. Walk about 3 miles east on highway 126. Then walk on gravel Foley Ridge Road about 10 miles to the Foley Ridge Trailhead. Then it's about 8 miles up to the PCT. This is just from looking at a map so "buyer beware".
• Eugene to Bend bus over Santiam Pass where the PCT crossses - they may let you out there and pick you up, you'de have to call them
• Olympic National Park Charter They would probably give you a ride from one trailhead to another, for example, Ozette to Rialto Beach.
• Craigs List can often get you a ride
• CET lists several transit companies between Eugene Portland Bend ...
• Point transit from Portland to Canon Beach/Astoria, Portland to Eugene

Make a request on oregonhikers.org and maybe you can get someone to give you a ride, especially from Portland to closer locations like Cascade Locks or Timberline Lodge.

Public Transit Hikes Hikes accessible by public transportation.