I haven't biked the coast, but want to at some point, probably N to S. About the trains:

I met some folks for a week at a house in Oregon, right on the coast. Train from Seattle to Eugene OR was a half day, and cheap, but that was my absolute limit, as the upholstered leather seats were still quite hard and flat. I met someone on the return trip, going from southern Cal to Seattle, and his words were "Never again!" Being able to sleep horizontally on my side in a cabin would be fine, but the price of that is way more expensive than flying. I could have hung a full size bike in the baggage car, but others have related when their train cargo had no hooks and the bike got flailed around in the baggage, or in under-car compartments not designed for bikes. I had a 20" wheel folder so avoided all that, Amtrak regs say you can bring that onboard the passenger car, and just put it on the luggage shelf; I had tons of room for that (end of covid period) and if I needed to, I could have stood the folded bike up vertically and covered with a garbage bag to keep oil off adjacent luggage. Panniers went in the overheads at the seat, but I was prepared to lash together and check in luggage, but never needed to.

I could have biked to and from the coast from Eugene, but there was a dirt-cheap shuttle bus run by the casinos; Did have front bike rack, but my front rack blocked the locking arm; No problem, just folded my bike and brought on board the short-bus. Was a problem: High passenger seating was above windshield height, and I got motion sick; Needed to sit on floor of bus to see out front and avoid that, very uncomfortable, but worked for the hour drive. Next time I will plan on biking.

Regarding bikes... Trek used to make a great tourer, the 520 I think, chrome-moly steel, great factory racks, haven't made in a few years. Masi had an awesome looking tourer, also chrome-moly, amazing looking welds, I think possibly laser, also great factory racks included, and bar-end shifters instead of brifters, IMO, more reliable for a tour, I don't know if they still make. Both were bargains at about USD$1500.

Pics of folded bike in train car, and on coast, this was not heavy touring but clothes and some food for a week, plus I wore about a 2000 cubic inch backpack, this was just for short biking to and from the train. Really, I brought the bike so I could bike all week, sans cargo.