WASHINGTON - Responding to a series of fiery train crashes, the government proposed rules Wednesday that would phase out tens of thousands of older tank cars that carry increasing quantities of crude oil and other highly flammable liquids through America's towns and cities, including Philadelphia.

Many details were put off until later as regulators struggled to balance safety against the economic benefits of a fracking boom that has sharply increased U.S. oil production. Among the issues: What type of tank cars will replace those being phased out, how fast will they be allowed to travel, and what kind of braking systems will they need?

Accident investigators have complained for decades that older tank cars, known as DOT-111s, are too easily punctured or ruptured, spilling their contents when derailed. Since 2008, there have been 10 significant derailments in the U.S. and Canada in which crude oil has spilled from ruptured tank cars, often igniting and resulting in huge fireballs. The worst was a runaway oil train that exploded in the Quebec town of Lac-Megantic a year ago, killing 47 people.

Transportation Secretary Anthony Foxx said he expected his department to complete final regulations before the end of the year. First, the public and affected industries will have an opportunity to comment on the proposal.

"We are at the dawn of a promising time for energy production in this country," Foxx said. "This is a positive development for our economy and for energy independence, but the responsibilities attached to this production are very serious."

Oil trains come to the Philadelphia region, feeding crude to refineries on the Delaware River.

In a report released along with the rules, the Department of Transportation concluded that oil from the Bakken region of North Dakota and Montana, where fracking methods have created an oil boom, is more volatile than is typical for light, sweet crudes.

The oil industry immediately challenged that conclusion. "The best science and data do not support recent speculation that crude oil from the Bakken presents greater than normal transportation risks," said American Petroleum Institute president and CEO Jack Gerard. The department "needs to get this right and make sure that its regulations are grounded in facts and sound science, not speculation."

Rail shipments of crude have skyrocketed from a few thousand carloads a decade ago to 434,000 carloads last year. The Bakken now produces more than one million barrels a day, and production is increasing.

The phase-in period for replacing or retrofitting older tank cars that transport the most-volatile types of liquids is shorter than the Canadian government's three-year phased plan. Congress, fearing another Lac-Megantic, has been pressuring regulators to put new safety rules in place as soon as possible.

Regulators also are weighing whether to limit crude and ethanol trains to a maximum 40 m.p.h. throughout the country, or just in "high-threat" urban areas or areas with populations greater than 100,000 people. A high-threat urban area is usually one or more cities surrounded by a 10-mile buffer zone.

Railroads had already voluntarily agreed to reduce oil-train speeds to 40 m.p.h. in urban areas beginning July 1. Tank cars - including the newer ones built to a tougher safety standard - have ruptured in several accidents at speeds below 30 m.p.h. Regulators said they were considering lowering the speed limit to 30 m.p.h. for trains that weren't equipped with advanced braking systems.

The freight-railroad industry had met privately with department and White House officials to lobby for keeping the speed limit at 40 m.p.h. in urban areas, rather than lowering it. Railroad officials say a 30-m.p.h. limit would tie up traffic across the country because other freight wouldn't be able to get past slower oil and ethanol trains.