Canada's Transport Minister Lisa Raitt speaks during an interview with Reuters in Ottawa March 25, 2015. REUTERS/Chris Wattie

TORONTO (Reuters) - Canadian officials are studying the impact of long oil trains on the integrity of the country’s railway tracks, Transport Minister Lisa Raitt said on Friday.

Raitt said the research is part of the federal transport department’s response to a series of derailments along Canadian National Railway Co’s mainline in northern Ontario earlier this year.

“We’re looking at oil unit trains,” Raitt told reporters in Toronto. “It’s a significant change in how we’re moving goods in the country, so that we should take a look at it.”

Oil unit trains are made up almost entirely of tank cars carrying crude oil rather than a mixture of freight.

Some industry experts have suggested that the trains might put unusual stress on railway tracks. Others point out that while oil unit trains are new, heavier unit trains carrying coal and other commodities have operated for decades.

CN Rail, the country’s biggest railway, suffered a string of derailments in February and March, including three along one section of its main route through northern Ontario. Two of those trains were carrying oil.

A Reuters investigation found CN Rail had suffered a spike in derailments last year as accidents caused by poor track conditions jumped. CN blamed the increase on bad weather and increased freight volume, and in April boosted spending on its track network.

A crackdown on oil unit trains could raise the cost of shipping Canadian crude by rail as unit trains are typically cheaper to operate than trains with mixed freight.