Amtrak had 31.3 million passengers in 2016 [1]. That's 0.095 passengers per capita. And that's on all Amtrak trains, while the German number is just for the ICE high-speed trains. Amtrak's high-speed Acela trains only had 3.5 million passengers in 2019 or 0.01 per capita. [2]
Per capita is just not a useful number for this discussion.
[1] https://www.amtrak.com/content/dam/projects/dotcom/english/p...
[2] https://www.statista.com/statistics/1155658/acela-high-speed...