There were 137 passengers and 11 crew members on board, Amtrak said. All have evacuated.
“There are no reported injuries to the crew or passengers,” Amtrak said in a statement to CNN.
Evan Courtney said he didn’t see or hear what happened, but he was in a lounge car when “people came running through yelling, ‘Shots fired!'”
Courtney said he then grabbed his backpack and ran. He said he huddled with strangers in a hallway, looking out the window.
“I looked out of the window and saw SWAT with assault rifles huddled behind barricades,” Courtney told CNN.
“After about 15 minutes, police ran to us and told us to get out of the car and run in the opposite direction, out of harm’s way.”
One person has been detained, the Tucson Police Department tweeted.
An earlier tweet by Tucson police said it was a “very active scene” and asked people to, “please avoid the area” on Toole Avenue, where the Amtrak station is located.
“Information is limited at this time, updates to follow,” the police department said.
Amtrak said it was “in contact with local and federal authorities regarding a shooting incident onboard Amtrak’s Sunset Limited train.”
The train, traveling from Los Angeles to New Orleans, had arrived at the station just before 8 a.m. local time Monday.
The motive for the shooting was not immediately clear.