A 27-year-old man has been arrested after a bottle containing hydrochloric acid was left on a train.
The suspect was detained on suspicion of endangering railway safety.
Middlesbrough station was evacuated and closed after a passenger reported a burning smell on board a Northern Rail service to Darlington on Sunday evening.
The hazardous liquid had been left in a drinks bottle in the middle of the train.
Emergency services, including firefighters and an ambulance, were called to the station as a precaution due to the fumes from the highly corrosive substance, and the train was also evacuated.
British Transport Police released two CCTV images of a man officers wanted to trace, and a Middlesbrough man was later arrested.
A spokeswoman said: "We would like to thank members of the public for coming forward with information, as well as the media for assisting with our appeal.
"The man remains in custody and is currently helping police with their inquiries."