Ten hours by road. Large enough to have sleeper compartments or reclining seats. (Large enough to have restrooms.) Driven by an NI -- natural intelligence -- whose competence is accredited by the state government in a reasonably fair examination of skills. Give a dedicated lane to the bus, and seven hours might be within reach.
Really, the problem is that I see flights between the two cities at $70 per person and 2.5 hours in the air. Even if it takes an hour on each extra, that's 5.5 hours versus 7 for a fairly reasonable price.
All these modes of transportation have pros and cons. How do we get people to take less flights? Busses already exist. Decent fast and cheap trains don't, nor do self-driving cars. Both will help poach people away from shorter flights, but one is (potentially) much cheaper and easier to implement.
I would hope that Amtrak in the long term also electrifies the rail network so that the dependency on diesel engines can be reduced - for some routes this may not make sense, but where you have frequent services it seems that it would be a worthwhile impovement too.
It's so easy to imagine taking a self-driving sleeper taxi overnight between Portland and Oakland in 10 hours - just throw a bed in the damn thing, go to sleep, and wake up at your destination. Give self-driving cars a dedicated lane with a different speed limit, and you could bomb the trip in 7 hours, easy. It'd all utilize existing infrastructure, we'd just be increasing the throughput.
If the car is electric, it'll be multiples cheaper than the fuel costs of driving yourself, a spiffy van with nice spread out reclining seats could have multiple passengers so the cost could be split further, and it'd probably be 1-2 orders of magnitude better for the environment than taking a flight.