The Transportation Safety Board (TSB) of Canada says the city decided not to install a type of heat-detection equipment on light rail trains, which could have warned of problems on the train that derailed in August.
The letter focuses on the bottoms of the two-vehicle trains, specifically the cartridge assemblies and large nut that attach the wheels to the axles.
Hours before the wheel derailed, the TSB says there was an axle failure on one vehicle that meant the operator would have seen multiple wheel slip warnings at about 1:25 p.m. — but they kept the train going.
After the train kept moving along the 13-stop line, the operator reported a burning smell at about 2:50 p.m. at its westernmost station, Tunney's Pasture.
Roller bearing parts found near uOttawa station around the middle of the line suggest a cartridge assembly failed at some point during those 85 minutes and those parts fell off, according to the TSB.
Back at Tunney's Pasture, a technician found burn marks on a brake disk on one vehicle, misidentified the problem and didn't check the train further, including the other vehicle.
From 8:15 p.m. to 8:25 p.m., technicians indicated the train moved smoothly back and forth on the tracks and cleared it to return to the maintenance yard, leaving the station at 8:30 p.m.
Almost immediately, two wheels derailed as it navigated a switch.
WATCH | August's LRT derailment:
Wheel 'severed' due to bearing failure, TSB says
When workers went to put the train back on the tracks they discovered a wheel was no longer attached.
"The No. 3 wheel had severed from the axle due to a previously undetected catastrophic roller bearing failure and subsequent axle journal burn-off."
This derailment shut down the Confederation Line for almost a week as the fleet was inspected and nine other vehicles were found to have wheel problems.
The city is now more closely inspecting underneath its light rail trains, according to the TSB, which points out Via Rail trains have systems that can measure the heat inside its wheels to warn of potential problems.
The City of Ottawa decided against a similar system because it believed regular maintenance would be sufficient. The TSB's letter said the city may want to reconsider this decision.
Without this system, the letter says "an overheated roller bearing within the cartridge assembly can potentially fail catastrophically without being observed or detected."
The city has not yet responded.
LRT currently shut down
The Confederation Line is currently shut down because another train derailed around Tremblay station Sept. 19. That too has triggered a TSB investigation, among other reactions.
No injuries were reported in either incident.
An advisory letter is part of the report process when inspectors find something it should pass along, but it is not the final report.
The regular 90-day timeline for these types of investigations would see the final report for August's derailment released in mid-November.