I wonder how much it would cost to add extra tracks in places like the Seattle -> Eugene line (that I recently took, except that the trip back turned into a bus

I wonder how much it would cost to add extra tracks in places like the Seattle -> Eugene line (that I recently took, except that the trip back turned into a bus due to ... well, freight I think). It's mostly emptiness, my understanding is that actually laying the track isn't hard. But the more I think about other rail networks, the more I feel like I almost never see multi-track lines. I think these lines are also being used for freight and yet... is the US really just running uniquely long trains for freight?