OTTAWA -- The federal transportation watchdog is investigating after Ottawa's LRT service was shut down Monday due to an axle becoming dislodged from the track.

The Transportation Safety Board has sent investigators to Tunney's Pasture station, it said in a news release Monday.

"The TSB will gather information and assess the occurrence," the release said.

In a statement to CTV News Ottawa, the TSB said it was told one of the train's wheels derailed while crossing tracks.

"At 8:34 p.m., 8 August 2021, an empty two-car Light Rail Transit O-Train was crossing from track 1 to track 2 at Tunney’s Pasture when one wheel on the one of the cars derailed on a switch. The train remained upright. There were no injuries and no dangerous goods involved. The switch was damaged," the statement said.

"We are continuing to gather information and assess the occurrence."

The closure of the system was made out of “an abundance of caution” to ensure all trains in the fleet continue to operate safely, OC Transpo head John Manconi said in a memo Monday morning.

On Sunday evening, an out-of-service train was leaving Tunney’s Pasture to return to the maintenance and storage facility when the operator experienced an “abnormal and rough ride,” the memo said.

The vehicle was found to have one axle out of 10 off the rail. It remains parked outside Tunney’s Pasture station.

Rideau Transit Maintenance is investigating what happened. Until the root cause is identified, rail service on the Confederation Line is suspended.

R1 replacement bus service is running until rail service is restored.

The TSB is also involved in a separate investigation into cracked wheels on the LRT. The TSB says that investigation, which began in July 2020, is in the report phase.

Director of Transit Operations Troy Charter told CTV News Ottawa that the issue could be related to one of the trains and not the track.

"Early indication is that it doesn’t appear to be track related, but the investigation was started immediately last night,” Charter said. “It does appear to potentially be a car issue.”

He noted that service reliability on the LRT has been very high in the past several months, though ridership remains well below pre-pandemic levels. OC Transpo said in a separate memo Monday that ridership was at approximately 30 per cent of pre-pandemic levels at the end of July.

"Unfortunately, like all rail systems, things can happen. Our focus right now is finding out what happened and getting back to service as safely and quickly and reliably as possible," Charter said.

He acknowledged that the suspension of service is an inconvenience to customers and likely means longer travel times for many riders.

"There definitely was an inconvenience to our riders and you know our customers are feeling that," he said. "I’m not going to minimize this. Our customers who rely on the service. They plan their itineraries, they plan their days around it, so any time you’re adding additional time for customers or any sort of disruption, that’s hard."

'Very concerning,' transit commissioner says

Citizen transit commissioner Sarah Wright-Gilbert says this latest incident is disturbing.

"I thought the wheels were disturbing, the issues with the wheels that the TSB is also investigating. This is worse, from my layperson's perspective," she said. "It's very concerning to me because while this incident was at a relatively lower speed and there were no passengers on board, this very easily could have happened at 80 km/h with a full train and that could have been worse."

Wright-Gilbert said she is pleased the TSB is involved, but added that she's concerned about how long it could take to find the root cause of the problem.

"How long are we going to be doing replacement buses when we don't have enough buses to provide service to the entire city because we got rid of them in lieu of the train?" she asked. "There are still people in the city of Ottawa who rely on public transit to support their everyday lives. This disruption in service is unacceptable and it absolutely disrupts their lives."

Charter said he did not want to speculate about how long the investigation might take, but suggested it could be "a few days."