Ending months of uncertainty and delays, federal regulators on Friday unveiled new rules for transporting crude oil by trains, saying the measures would improve rail safety and reduce the risks of a catastrophic event.

But the rules quickly came under criticism from many sides. Lawmakers and safety advocates said the regulations did not go far enough in protecting the public, while industry representatives said some provisions would be costly and yield few safety benefits.

More than two years in the making, the rules followed a spate of derailments, explosions and oil spills around the country that highlighted the hazards of shipping large quantities of potentially explosive material on rails. The regulations introduce a new tank car standard for oil and ethanol with better protections, and mandate the use of electronically controlled brakes.

Facing growing pressure from members of Congress as well as local and state officials, the Department of Transportation has taken repeated steps in the last two years to tackle the safety of oil trains and reassure the public. Last month, for example, it set lower speed limits for oil trains going through urban areas.

Under the new rules, the oldest, least safe tank cars would be replaced within three years with new cars that have thicker shells, higher safety shields and better fire protection. A later generation of tank cars, built since 2011 with more safety features, will have to be retrofitted or replaced by 2020.

Oil trains — with as many as 120 cars — have become common sights in cities like Philadelphia, Albany and Chicago as they make the slow journey from the Bakken region of North Dakota, where oil production has surged in recent years.

Local and state officials have complained that rail-friendly rules make it difficult to predict when trains will pass through.

But regulators retreated from a provision that would have forced railroads to notify communities of any oil train traffic. Instead, railroads will need to have only a “point of contact” for information related to the routing of hazardous materials.

Several members of Congress, particularly those representing states like Washington, Oregon, North Dakota and New York that have seen a surge in train traffic, said the rules did not go far enough and signaled that legislation might be needed.

Senators Ron Wyden and Jeff Merkley of Oregon said they were disappointed that transportation officials had not expanded public information about oil train routes.

“Instead of providing first responders more details about oil shipments, railroads will simply be required to give our firefighters a phone number,” they said.

Railroads said they welcomed the new regulations but objected to a provision that would require tank cars to have electronically controlled pneumatic brakes by 2021. The Department of Transportation said the new brakes, known as E.C.P., are more effective than air brakes or dynamic brakes that are currently being used.

“The D.O.T. couldn’t make a safety case for E.C.P. but forged ahead anyway,” Edward R. Hamberger, the president and chief executive of the Association of American Railroads, said in a statement. “I have a hard time believing the determination to impose E.C.P. brakes is anything but a rash rush to judgment.”

The railroad association has estimated in comments filed to the Transportation Department last year that installing the new brakes would cost $9,665 per tank car. The Railway Supply Institute, which represents tank car makers, also pushed against the use of those brakes, saying their effectiveness was not proved and would not provide a significant safety advantage.

Transportation officials said the new type of brakes was already in use by some railroads for other types of commodities. Their use would decrease the chances of a catastrophic pileup, reduce the number of punctured cars in an accident, or allow train operators to stop faster if there was an obstacle on the tracks.

Sarah Feinberg, the acting administrator of the Federal Railroad Administration, said: “The mission of the F.R.A. is safety and not focusing on what is convenient or inexpensive or provides the most cost savings for the rail industry. When I focus on safety, I land on E.C.P. It’s a very black-and-white issue for me.”

There have been five explosions and spills this year alone, four in the United States and one in Canada. In July 2013, 47 people died in Canada after a runaway train derailed and exploded in the city of Lac-Mégantic, Quebec.

“I am hopeful the rail industry will accept this rule, and will follow this rule,” Anthony Foxx, the transportation secretary, said at a news conference in Washington. He appeared with Canada’s transport minister, Lisa Raitt, who said Canadian and American regulations would be aligned.

A central question before the administration was to determine what level of protection the new generation of cars should have and how quickly to roll them out.

The new rules create a new standard, “high-hazard flammable trains,” defined as “a continuous block of 20 or more tank cars loaded with flammable liquid or 35 or more tank cars loaded with a flammable liquid dispersed through a train.”

By 2018, the rule would phase out older tank cars, DOT-111s, long known to be ill suited for transporting flammable material. A newer generation of cars, known as CPC-1232, would have to be retired or refitted to meet the new standard, DOT-117, by 2020.

All cars built under the DOT-117 standard after Oct. 1, 2015, will have a thicker nine-sixteenths-inch tank shell, a one-half-inch shield running the full height of the front and back of a tank car, thermal protection and improved pressure-relief valves and bottom outlet valves.

Senator Charles E. Schumer, Democrat of New York, said Friday’s announcement gave railroads too much time to remove older cars from service. Mr. Schumer was one of seven senators who unveiled a bill that would seek to impose a fee of $175 per shipment on older cars to speed up their removal from service.

“The good news is that the standards are predictable, but the bad news is that the phaseout time is too lenient,” Mr. Schumer said.

Senator Marie Cantwell, Democrat of Washington, was more forceful, saying that the new regulations also failed to reduce the volatility of Bakken crude, which is more likely to catch fire and explode than other forms of crude.

“It does nothing to address explosive volatility, very little to reduce the threat of rail car punctures, and is too slow on the removal of the most dangerous cars,” she said. “It’s more of a status quo rule.”

Oil companies, though, said the mandate to build new tank cars to replace older models starting in 2018 would stretch the industry’s manufacturing ability and lead to shortages.

Placing blame on the railroads, Jack Gerard, the chief executive of the American Petroleum Institute, said regulators should focus instead on preventing derailments and enhancing track inspection and maintenance.

The spectacular growth of oil production from the Bakken region, negligible only a few years ago and now exceeding a million barrels a day, has transformed the domestic energy industry. It has placed the United States back on a path to oil self-sufficiency, and profoundly disrupted international energy markets.