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 as far East as Rhododendron. Just before that is a stop at Welches Road. You could walk about a mile on the road to the Zigzag Mountain Trail, and from there get up to Mount Hood and the PCT. Or stop at Mt Hood Village and hike over to the Wildwood Recreation Site and up the Boulder Ridge Trail into the Salmon-Huckleberry Wilderness. July 2014 they added a bus to Timberline Lodge - 5 times a day year round. It would require a bunch of transfers to get from Portland to Timberline Lodge.
• 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 Public Transit from Vancouver to North Bonneville, Stevenson, and Carson
• 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 15 miles from the PCT and the Three Sisters
• 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

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.