Highly volatile Bakken crude oil is carried by train from North Dakota to refineries on both coasts of the U.S. and pass through major cities including Chicago, Philadelphia and Seattle. That creates conditions for a deadly disasters if something goes wrong.

PHILADELPHIA — They rumble past schools, homes and businesses in dozens of cities around the country — 100-car trains loaded with crude oil from the Upper Midwest.

While railroads have long carried hazardous materials through congested urban areas, cities are now scrambling to formulate emergency plans and to train firefighters amid the latest safety threat: a fiftyfold increase in crude shipments that critics say has put millions of people living or working near the tracks at heightened risk of derailment, fire and explosion.

After a series of fiery crashes, The Associated Press conducted a survey of nearly a dozen big cities that, collectively, see thousands of tank cars each week, revealing a patchwork of preparedness. Some have plans specifically for oil trains; others do not. Some fire departments have trained for an oil train disaster; others say they’re planning on it. Some cities are sitting on huge quantities of fire-suppressing foam, others report much smaller stockpiles.

The AP surveyed emergency management departments in Chicago; Philadelphia; Seattle; Cleveland; Minneapolis; Milwaukee; Pittsburgh; New Orleans; Sacramento, California; Newark, New Jersey; and Buffalo, New York. The responses show emergency planning remains a work in progress even as crude has become one of the nation’s most common hazardous materials transported by rail. Railroads carried some 500,000 carloads last year, up from 9,500 in 2008.

“There could be a huge loss of life if we have a derailment, spill and fire next to a heavily populated area or event,” said Wayne Senter, executive director of the Washington state association of fire chiefs. “That’s what keeps us up at night.”

The oil comes from North Dakota’s prolific Bakken Shale, an underground rock formation where fracking and horizontal drilling have allowed energy companies to tap previously inaccessible reserves.

The production boom has made oil trains a daily fact of life in places like Philadelphia, where they roll past major hospitals, including one for children. In Seattle, they snake by sports stadiums used by the Seahawks and Mariners before entering a 110-year-old tunnel under downtown. In Chicago, they’re a stone’s throw from large apartment buildings, a busy expressway and the White Sox’s ballpark.

Before the rise of shale oil and the ethanol industry, hazardous materials were typically shipped in just a handful of cars in trains that hauled a variety of products. But the trains now passing through cities consist entirely of tank cars filled with flammable crude. These so-called unit trains offer increased efficiency but magnify the risk that hazardous materials will be involved in a derailment.

That has led some residents and emergency management experts to worry it’s just a matter of time before a catastrophic derailment in a city, where, according to a 2014 U.S. Department of Transportation analysis, a severe accident could kill more than 200 people and cause $6 billion in damage.

Two summers ago, an oil-train derailment, explosion and fire showed the power of such a disaster in even a small town, when part of Lac-Megantic, Quebec, was leveled and 47 people died. There have been at least six oil-train derailments in lightly populated areas of the U.S. and Canada so far this year, most resulting in fires but none in deaths.