Other Amtrak passengers are reporting more difficult encounters, often involving drug enforcement and civil forfeiture (a cynic would say, the government reverting to its primeval warlord form, follow the link for additional commentary) and wee-hours search tactics that rise to the level of unreasonable searches and seizures.
An Amtrak official responded to my inquiry about the Texas Eagle encounter. The salient paragraphs follow.
Generally, Amtrak does support outside law enforcement agency efforts when they are on Amtrak property as long as they are engaged in bona fide and legal law enforcement operations. Amtrak operates a railroad route system that is about twenty thousand miles (20,000) and the Amtrak Police Department needs assistance from such agencies to help keep Amtrak passengers, employees and property safe and secure.Note in the middle paragraph the hedged "does not have to consent to talk ... and engage in a voluntary encounter." That's easier to insist on and adhere to in the middle of the day than it would be in the wee hours, which appear to be the preferred time for the drug enforcement chekisti and their sniffer dogs to do their work. As Reason's Brian Doherty notes, "gendarmes bothering innocent travelers on trains is a storytelling cultural sign of a malign, sinister European/banana republic tyranny in the world I grew up in."
Here I would agree with you that a passenger does not have to consent to talk to a law enforcement official and engage in a voluntary encounter if the passenger does not choose to do so. If a passenger does elect to continue to speak with law enforcement officials and answer questions, however, then that officer can continue the communication with any passenger or citizen.
Amtrak does utilize various police and security practices, including random ticket verifications. However, they are performed by Amtrak employees. At this time, I am attempting to verify the agency that conducted this activity and the reason for it.
Time to turn "if you see something, say something" around. Conor Friedersdorf asks readers to report searches they've seen to him, and the train enthusiast discussion boards do some keeping track. A government cannot hassle and intimidate a population that will not be hassled and intimidated.