I've cycled from numerous places in Oregon to the coast. We did it in reverse on a bike trip ending in Astoria last summer. Amtrack runs a bus between PDX and Astoria. The only hassle is the bike needs to be boxed to go on the bus. We had to box the bikes to go from PDX to Vancouver, BC; but he driver in Astoria let us put the bike on the bus unboxed (it was an early Sunday morning). If your bike is already boxed not a problem. Portland station sells boxes for $15. They are big boxes, and very little disassembly is required.

Good advice above. Hwy 30 is a good ride along the Columbia River. Salem to Lincoln City via of Highway 22-18 is a zoo, The Jewel turnoff from 26 to Astoria has light traffic, but is narrow with lots of curves, and log trucks.
We've ridden Hwy 20 from Albany to Newport and it is narrow but OK. There is a train station in Albany. My choices would be Hwy 30 or Hwy 20, if I was starting from a train station in the Willamette Valley.