Hmm, looks like the data I cited was a bit dated. Amtrak mentions 94% for their 2016 fiscal year [1], which isn't bad.
[1] https://media.amtrak.com/2016/11/amtrak-delivers-strong-fy-2...
[1] https://media.amtrak.com/2016/11/amtrak-delivers-strong-fy-2...
What if a specific National Park, arguably of equal value to the Amtrak lines, generated 94% of its own money?