For example, Portland to Seattle isn't that far but I-5 can easily back up and become an hours-long ordeal, and SEA and PDX aren't particularly close to a lot of places.
Agreed that SEA to PDX is uniquely useful on the west coast. Similar trips are also practical in the north east for the same reasons (relatively short distance for a flight, high car traffic and great station locations)
While this is true, there is also something to be said about predictability.
A train that is an hour slower than the best driving time but consistently so is easier to plan around than a car that is an hour faster or an hour slower than the train with no clear rhyme or reason.
For example, Portland to Seattle isn't that far but I-5 can easily back up and become an hours-long ordeal, and SEA and PDX aren't particularly close to a lot of places.