A northern Ontario spill of oil from a derailed train is 100 times larger than Canadian Pacific Railway Ltd. initially reported.

The company said Wednesday that only four barrels spilled. On Thursday, it said some oil had flowed beneath the snow and gone undetected. CP now estimates 400 barrels spilled, or 63,500 litres – a slightly greater amount than the company's spill last week in Minnesota.

At about 7:50 Wednesday morning, 22 rail cars derailed about 10 kilometres west of White River, a small northern Ontario town. Two of those cars leaked light oil.

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"The original leaking car was secured. The second car … showed no signs of the product around its base during initial assessments," CP spokesman Ed Greenberg said.

When the leak was discovered under the snow, "CP constructed a berm and other containment and mitigation equipment is in place."

The company said it does not believe the oil leaked beyond containment booms or into water, although testing is in place.

Asked how CP could miss such a large amount of leaked oil, Mr. Greenberg said workers initially believed the leaking had stopped at the second car.

"Appreciate the conditions that our crews are working in at a derailment site. … In the second car it was difficult to assess its condition due to its position among the derailed equipment," he said.

CP resumed service on its tracks near White River, a major company line, Thursday evening.

The spill comes as pipeline shortages push growing volumes of oil onto trains. Rail tank cars are now moving oil across the continent from both the Bakken oil play in North Dakota and, increasingly, from both light and heavy oil fields in Canada.

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The advent of rail transport has sparked a debate about the relative safety of oil pipelines compared to trains. Industry statistics have shown that trains have more spills. The rail industry argues that its spills are smaller, and it therefore spills less than pipelines. Some statistics show otherwise, however, and pipeline companies have argued that buried pipe is the safest method. Backers of rail movements, meanwhile, have argued that heavy oil – the kind that flows from the oil sands – can be moved more safely on tracks.

The oil and gas industry has faced intense scrutiny following major safety breaches in the Gulf of Mexico and Kalamazoo River. Subsequent pipeline spills – including a 12,000-barrel leak from an Exxon Mobil Corp. pipeline in Arkansas in late March – have provided fodder to critics who say new pipelines, including TransCanada Corp.'s Keystone XL project, are risky propositions.

Industry, however, says it safely delivers more than 99.99 per cent of the barrels that move both through pipelines and on trains.