Calgary police say foul play is not suspected after a body was found on board a C-Train in the city's northeast.
They say security staff discovered a woman slumped over in her chair and called 911.
EMS pronounced the woman dead and now police are reviewing CCTV to try and figure out what happened.
It's believed she may have had a heart attack.
Police say it appears two men may have been with or close to the woman on the train.
One of the men was located and is being questioned in connection to the incident.
Police are still looking to speak with the second man.
The Whitehorn C-Train station is closed for now but trains are still running.