Police met the California Zephyr train about 22 miles west of Denver late Sunday after the crew alerted authorities, Amtrak spokeswoman Karina Romero said.
CBS Station KCNC correspondent Michelle Griego reported that a male passenger was overheard making nonspecific threats, which concerned officials enough to stop the train.
218 people were evacuated and loaded onto buses to Denver's Union Station.
Authorities detained the man for questioning and did a sweep of the baggage car and the passenger car where he was sitting. The man was carrying a Bowie knife, and a bomb-sniffing dog focused on his bag, Romero said.
"There was something that was in that bag that made the dog stop," Romero said, declining to elaborate.
No injuries were reported.
Passengers were loaded onto buses and brought to Union Station in Denver. Those passengers who were destined fore Denver had to then wait before they could go back to the train to retrieve their belongings. Passengers destined for Chicago eventually resumed their journey.