What are the legal options of a person asked this question?
Can I just give them a stank eye? Can I request to see an identification badge? Can I take still pictures of them? I can I record video of them?
Um, need answer fast… 
Whoop whoop
Sir Topham Hatt?
…
The Internet Tough Guy in me would want to say “eat shit, racist”. The clever me would say something really pithy about Nazi Germany and “papers”, but the real me probably just says “yes” too.
It’s probably not something that happens on every train, but it’s been a regular thing. Here they are nabbing someone off the Empire Builder in Havre in 2013 based on a Border Patrol check. Here is one from 2015. Here is one from 2010.
My earlier post linked to a Nation article about Border Patrol enforcement on the Empire Builder in Havre in 2005. It apparently even predates 9/11: Here is an arrest off the Empire Builder in Havre in 2000, and that article mentions that 83 people were arrested off Amtrak in Havre during 1999-2000.
Fucking dragnets, how do they work?
:D:D:D:D
I’ve used it before. Tragically, I can no longer remember the joke alluded to here, which was probably just as good.
It is my understanding that they (ICE) have no jurisdiction if you are over 100 miles from the border.
Nope. ICE can operate anywhere. The Border patrol is limited to within 100 miles of the border. Havre is within that limit anyway. So are two thirds of Americans.
I thought the proper response was claim to be a sovereign citizen and ask “Am I being detained?”
I guess a response of “Qué?” would not be helpful.
(I visualise Manuel from Fawlty Towers)
Only if you remember to ¿
“Are you a citizen?”
“Yo soy Americano”
I daresay it would be mistake to reply to
“Are you an American citizen?”
with
“Jawohl, Herr Gruppenfuhrer!”
It depends on the agent.