The Transportation Safety Board of Canada (TSB) says the September derailment of an Ottawa light rail train was caused by improperly tightened bolts, according to a letter sent to the city on Tuesday.

In a memo to city council and the transit commission, city manager Steve Kanellakos said the city agrees with the board's findings.

"The root cause of the derailment was due to inconsistent and incomplete maintenance of the light rail vehicle components by Rideau Transit Maintenance (RTM) at the maintenance facility," he wrote.

The LRT network has been shut down since Sept. 19 when a train derailed near Tremblay station, the second derailment on the line in roughly six weeks.

When servicing trains after the Aug. 8 derailment, staff at the RTM maintenance facility did not properly document their work, which led to a train being put back into service with loose bolts connecting the gearbox to the bogie, which holds the axles and wheels.

All 12 of the bolts in the replacement part failed after the train was only in service for 800 kilometres.

The bolts in question needed to be torqued with an electronic gun, and while there were some checklists, there were none for all of the necessary safety steps.

Because maintenance steps were not followed in a consistent order and the paperwork was not adequately filled out, steps were accidentally skipped during a shift change.

There was also a lack of quality control and oversight at the time, the TSB found.

"A robust quality control system features ... checklists for the assembly or reassembly of safety critical components. Each step should be documented, signed off on and verified by supervisory staff," the TSB wrote. "This is particularly important when a job is started by one work shift and handed off to another."

The third-party review by Transportation Resource Associates (TRA) came to a similar conclusion, according to Kanellakos.

The TSB letter recommends an in-depth review of all work on important safety components for the trains, which Kanellakos says is part of the TRA's review of the Rideau Transit Group's return-to-service plan.

"TRA has been working with us to oversee RTM's inspection work and ensure it follows best practices for the safe operation of LRT. We expect nothing less from RTG, RTM and Alstom going forward," said a spokesperson for Mayor Jim Watson in an email.

Details of that return-to-service plan are set to be revealed at a briefing on Friday morning.