Freight lines have sharper curves then passenger lines. Freight cars are shorter than passenger cars which allows them to trail over shorter radii rail curves.
Yes, I know that, which is why I pointed out those two curves on either side of I-5. Seeing as how that stretch is repurposed track, did Amtrak or BN run a safety study to establish speed restrictions, or that they could even move those long cars through that track?
A lot of industry track and sections like that have restrictions on what maximum car length you can run, speed, and can only run 4 axle locomotives on them.