CORRECTION: Some information attributed to James R. Beardsley of Marsh Inc. was omitted during the editing process in the original version of this story. It has been added below.

Freight railroads and their insurers are re-examining the catastrophic potential of rising crude oil shipments in the wake of July's deadly derailment and explosion of a runaway train in Quebec.

The accident, which killed 47 people and destroyed much of central Lac-Mégantic, Quebec, could produce up to $750 million in liabilities ranging from wrongful death claims to large pollution cleanup costs, sources said (see related story). It also pushed Montreal, Maine & Atlantic Railway Inc., the railroad involved, into bankruptcy reorganization.

The disaster highlights exposures in the fast-rising use of railroads to transport crude oil from newly developed fields such as those in North Dakota, particularly on smaller regional and local shortline railroads, insurance sources said.

It is also focusing railroads' and regulators' attention on those shipping risks, said Daniel Bancroft, senior vice president and transportation practice leader at Willis North America Inc. in New York.

Most U.S. crude oil is transported by pipeline and wasn't a large-volume commodity for railroads until recently. But shale drilling in North Dakota's Bakken formation created an opening for railroads, initially because of inadequate pipeline capacity and later because they could deliver oil to coastal locations that brought higher prices.

In the first quarter of 2010, U.S. Class I railroads, including U.S. units of Canadian railroads, originated 3,395 carloads of crude oil; by the first quarter of 2013, that number had rocketed to 97,135 carloads, according to the Washington-based Association of American Railroads.

Class I railroads — including BNSF Railway Co., Canadian National Railway, Canadian Pacific Railway Ltd., CSX Corp. and Union Pacific Corp. — carry most crude shipments from origination to final destination. Some shipments originate on smaller railroads or are turned over to them for deliveries on their networks.

The ill-fated MMA train, consisting of 72 crude oil tank cars and a boxcar, began as a Canadian Pacific train in North Dakota and turned over to MMA near Montreal. Operated by a single engineer, the 10,000-ton train was left unattended in Nantes, Quebec, after a small fire on one engine was put out; its brakes later failed to hold it, and it rolled downhill into Lac-Mégantic.

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Canadian regulators subsequently ordered that all trains carrying hazardous substances have two operators and not be left unattended on a mainline track.

The U.S. Federal Railroad Administration, meanwhile, issued an emergency order that trains with dangerous cargo not be left unattended unless a railroad adopts a specific plan, and ordered railroads to adopt plans to secure unattended trains. It also said its inspections of railroads since January 2010 showed “significant noncompliance” with existing regulations.

The Association of American Railroads said in a statement that its members will implement the emergency order, and railroads will review their operating procedures.

Industry experts note that railroads' overall safety record is excellent, and that they carry many substances more dangerous than oil, from chlorine to anhydrous ammonia, though some say the rapid growth of crude oil-by-rail is making railroads riskier.

That growth “absolutely increases the potential of a catastrophic loss,” said Nick Bayliss, assistant vice president with Gemini Transportation Underwriters, a Boston-based unit of W.R. Berkeley Corp.

And while Class I railroads have longstanding experience with handling hazardous materials, smaller regional and shortline railroads face more challenges, observers say. Until recently, for example, smaller railroads didn't usually handle heavy “unit trains” made up entirely of cars carrying one commodity, such as oil, that require much closer management, said Bill Anderson, president of Rail Services Inc., a railroad safety and claims consultant in Boise, Idaho.

Track and other infrastructure on smaller railroads also may not be up to the maintenance standards of Class I carriers, said James R. Beardsley, managing director and global rail practice leader with Marsh Inc. in Washington. On the positive side, he added, trains on these tracks move more slowly, cutting the risk of a large-scale derailment.

And for many smaller railroads, risk management practices haven't improved despite increases in self-insured retentions in recent years, meaning even basic steps such as securing engine cabs against trespassers aren't always followed, Mr. Anderson said.

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Smaller, local railroads carry relatively small liability limits: MMA had only $25 million in liability coverage with XL Insurance Co. Ltd., sources said. For a railroad carrying high-risk cargo, “$25 million probably isn't even prudent, much less adequate,” said Mr. Anderson.

In contrast, the eight U.S. and Canadian Class I railroads generally carry more than $1 billion in liability coverage, he and other rail experts said. Class I railroads buy available market limits up to $1.5 billion excess of retentions of $25 million to $50 million or more, insurance sources said.

Class II regional railroads typically buy $25 million to $50 million in limits, with some buying up to $100 million, while shortlines may buy as little as $5 million to $10 million, said Dave Adamczyk, vice president in the railroad department of Liberty International Underwriters in Hunt Valley, Md.

Several railroads have reviewed their exposures since the accident and have inquired about upping limits, market sources said.

Their decisions will be a “balancing act” between perceived risks and cost, said Mr. Beardsley. “They are looking at this exposure more on a catastrophic basis, then they'll make a decision on what they want to pay.”