"That's what concerns me more because it can be planned to do the worst possible damage," he told reporters after a speech to the Canadian Club of Toronto.
He said terrorism, or any third-party intervention, is a problem the company has "a hard time dealing with alone."
"That's not only somebody trying to do something to a hazardous tank, it's with somebody tampering with the railroads or people on the right-of-way," he said.
In Toronto, two men are facing multiple terror-related charges in an alleged plot to derail a passenger train travelling from New York to Toronto.
However, derailments are far more frequent than known terrorist threats.
Over the past month, at least two trains carrying crude oil derailed and caught fire — one owned by CSX Corp. in West Virginia and the other a Canadian National (TSX:CNR) freight train near Timmins, Ont.
Derailments of tankers carrying hazardous materials has been a focus as U.S. and Canadian officials discuss new tank car design standards, following the Lac Megantic, Que. explosion in 2013.
The derailment of the crude-laden train killed 47 people and burned the town's downtown core to the ground.
Some municipalities have pushed for railways to notify them when hazardous materials are moving through their cities. Harrison said while he's willing to provide those details, he's concerned they could wind up in the wrong hands.
"(If) you want to give someone the opportunity to break that custody chain and look at that list, and say 'Here's what that cars got in it, and here's the location and here's all the bad things we can do,' I don't think we want that," he said.
Canada and the U.S. have focused on the need for stronger tank cars, better brake systems and other safety improvements. Transport Canada is working with the U.S. government on a next-generation tank car to replace the 1232 standard car for the transportation of flammable liquids.
Harrison said more could be done to improve the safety of railway cars, but bureaucracy is stalling progress.
"It's time to get on with this," he said during his speech. "We can certainly minimize and improve the exposure that hauling these hazardous materials presents to the public."
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