The federal agency found a total of 254 incidents involving Canadian National (TSX:CNR), Canadian Pacific (TSX:CP) and Montreal, Maine & Atlantic went unreported — or were reported late — over a seven-year period.
Most of the incidents were considered minor, but the TSB's chief operating officer warned railways must still meet their obligations.
"We've made it clear to them that we expect them to fully comply with the regulations," Jean Laporte said in an interview.
"And if they don't, we're serious about this, we will consider taking action if required in the future ... We'll be keeping an eye and making sure everybody plays by the rules going forward."
The review comes as the rail industry faces heightened scrutiny following the fiery July 2013 derailment in Lac-Megantic, Que., which killed 47 people and wiped out part of the town. An unattended MMA tanker train loaded with volatile crude oil broke loose from its parking spot, roared down a hill and slammed into the community.
The disaster has forced regulators to grapple with safety concerns amid an ongoing, pan-continental boom in oil-by-rail shipments.
In its review Monday, the TSB found no injuries in any of the unreported incidents and described most of them as minor, often causing limited damage and usually confined to rail yards. The agency said it was lacking data from a range of categories, including those covering minor derailments and collisions.
Laporte said the reporting gap was due, in part, to changes last summer to the wording of the rule outlining what kinds of incidents must be reported. Until the July adjustment clarified reporting requirements, he said there were different interpretations of the regulations.
On Monday, Canada's two largest railways said they've adjusted their reporting methods.
The TSB found CP failed to report about 100 additional events, all of which took place over a 13-month period that started in January 2013. It said most of the unreported incidents inflicted only limited damage, though 15 of them were classified as leakage of a dangerous good.
"When TSB reporting regulations were changed in July 2014, CP responded quickly and is now fully compliant with the current regulatory requirements," CP spokesman Martin Cej said in an email Monday
"CP worked closely with the TSB to address the gap in reporting. CP's new processes and systems are fully in place now and CP continues to work closely with the TSB."
The TSB also identified 132 additional events at CN over the past seven years, including eight unreported incidents of "runaway rolling stock" between 2009 and 2013.
A CN spokesman said the company was aware of the unreported incidents and used them to develop internal safety programs.
"All were very minor derailments with very minimal damage, and none involved dangerous goods," Mark Hallman said.
"CN will continue to focus on every safety incident as a leading indicator of potentially more serious accidents."
In the case of the now-bankrupt MMA, the TSB uncovered 22 additional incidents between 2010 and 2013. The review listed two of the events as runaway rolling stock and said three of them involved dangerous goods.
The TSB review comes a few months after the agency blamed a lack of federal oversight and the penny-pinching MMA as contributors to the Lac-Megantic disaster.
In August, following its year-long investigation, the agency identified 18 key factors it says led to the catastrophe. It also made recommendations, including one urging Transport Canada to ensure railway safety systems are adequate.
At the time, then-TSB chair Wendy Tadros said the booming oil-by-rail sector ran "largely unchecked" across the continent.
Transport Minister Lisa Raitt responded quickly to the August TSB report by saying her department would review it and develop "concrete actions" to address the recommendations.
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