Ottawa faces a "threat to safe railway operations" after the O-Train sped past a stop signal without authority on two separate occasions in 2015, Transport Canada says.
According to a notice sent by the federal agency to the City of Ottawa last week, the two incidents happened on Nov. 18 and Dec. 24.
The first incident occurred near the junction of the Trillium Line and the city's Via Rail line, while the second happened near Gladstone Avenue.
In both cases, the trains passed the stop signals shortly after applying their emergency brakes, said Transport Canada.
"The frequency of emergency brake application events is affecting the situational awareness of O-Train operators and as a result is compromising safe operating practices," wrote Transport Canada railway safety inspector Mike Melville in the Jan. 20 notice.
"As a result of these findings, this notice is being issued ... to inform you that, in my opinion, a threat to safe railway operations exists."
There were no injuries and no damage as a result of either incident, according to the city.
First reported incidents in 14 years: city
The two occasions are the "first reported incidents of this nature in the Trillium Line's 14-year history," city manager Kent Kirkpatrick said in a Friday memo to city council.
Ottawa Mayor Jim Watson echoed that message in an interview on Saturday.
"We don't want to have any incidents, we want to make sure all of the systems are working," he said.
"In both cases, the systems and the brake system did work. But we have to make sure the operators are following the rules and we don't have any of these incidents in the future."
Trains on the Trillium Line automatically engage their emergency brakes whenever there's a "required safety response," and the fact they're engaged doesn't necessarily mean there's any danger, said Kirkpatrick.
A train travelling as little as 1 km/h outside of its prescribed range could have its emergency brakes activated, Kirkpatrick said.
City staff reported both incidents to the "relevant rail authorities," said Kirkpatrick.
The city has until Feb. 5 to tell Transport Canada how it plans to handle the situation.
Kirkpatrick said Capital Railway, which operates the O-Train, was already reviewing its emergency braking procedure and has put in more safeguards that go above and beyond Transport Canada requirements.