The U.S. Department of Transportation estimates trains carrying crude oil or ethanol will derail an average of ten times every year during the next two decades. Derailments are predicted to cause more than $4 billion in damage and possibly kill hundreds of people if an accident happens in a densely populated part of the U.S.

This previously unreported DOT analysis reviewed the risks of moving quantities of both fuels across the nation and through major cities. The study was completed in July 2014.

The volume of flammable liquids transported by rail has increased dramatically during the past decade, due mostly to the oil shale boom in North Dakotaand Montana. This year, rails are expected to transport nearly 900,000 car loads of oil and ethanol in tankers. Each can hold 30,000 gallons of fuel.

DOT based its foreboding forecast on past accident trends, anticipated shipping volumes and known ethanol and crude rail routes. About 15 derailments are predicted for 2015, declining to about five a year by 2034.

A total of 207 derailments in the 20-year period would cause $4.5 billion in damage, according to the analysis, which predicts ten "higher consequence events" causing more extensive damage and potential fatalities.

If one high-consequence derailment occurred in a high-population area, it could kill more than 200 people and cause roughly $6 billion in damage, the analysis concluded.

To get to refineries on the East and West coasts and the Gulf of Mexico, oil shipments travel through more than 400 counties, including major metropolitan areas such as Philadelphia, Seattle, Chicago, Newark and dozens of other cities, according to routing information obtained by The Associated Press through public record requests filed with more than two dozen states.

Since 2006, the U.S. and Canada have seen at least 21 oil-train accidents and 33 ethanol train accidents involving a fire, derailment or significant amount of fuel spilled, according to federal accident records reviewed by the AP.

The rail industry's overall safety record steadily improved in the past decade, dropping from more than 3,000 accidents annually to fewer than 2,000 in 2013, the most recent year available, according to the Federal Railroad Administration.

Railroads last year voluntarily agreed to reduce oil train speeds to 40 mph in urban areas. Regulators said they are considering lowering the speed limit to 30 mph for trains not equipped with advanced braking systems.