State population density simply doesn't matter. City population and distance between cities matters. Nobody realistically expects train lines to enable transportation to every square mile of the state.
You are correct. Most of Washington state is empty. Few people live in the Cascades or the eastern plains.
Most of the population of Washington and Portland Metro live inside a long rectangle with an area of 10-15000 square miles. Call it 400 people per square mile. Or probably 15000 people per mile of track which likely is a better number.
