WASHINGTON - Runaway oil production could slow road traffic as drivers face longer delays to cross train tracks in many congested regions, a U.S. study released on Friday predicted.

Oil, coal and grain shipments are taxing the national rail grid as the deliveries of those commodities are expected to climb along with commercial shipments in the coming years, according to the report from the Government Accountability Office, an investigative arm of Congress.

Freight movements on the tracks are due to rise 51 percent over 2007 levels by 2040, according to the Transportation Department, and so exceed 28 billion tons per year.

One factor is oil train deliveries out of North Dakota’s energy patch that neared 250,000 carloads in 2012 compared with roughly 10,000 in 2007, according to the study.

The increased oil shipments will translate into tie-ups at highway-rail crossings, though the study said it was hard to judge what areas of the country would be most affected.

But long trains blocking roads will come as a cost to quality of life in many parts of the country, according to the report.

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