Maloney said the railroad has also taken steps to prevent future derailments. After a 2010 incident in which 13 cars left the tracks in and near the tunnel, CSX agreed to replace track well ahead of schedule. It stepped up the frequency of its inspections, and agreed to deal directly with the city on information-sharing rather than requiring Baltimore officials to go through the state.

But for activists such as Fred Millar, an environmental consultant, that's not enough. He said it makes no sense to bring potentially lethal cargo through major urban centers.

"If they're still bringing chlorine into the city, you should be real upset about this," Millar said. He contends that federal authorities should require railroads to reroute such shipments through less populated areas — which in the case of CSX could require it to divert north-south East Coast traffic to its tracks through Ohio.

Millar does concede that some major users of the most dangerous chemicals have switched to less lethal products. Many water treatment plants, he noted, have switched from using chlorine gas to less dangerous bleach.

But that's not enough to keep the population safe, he said.

"We're still at risk, even if they do half as much as they used to," he said.

Rick Hind, legislative director for Greenpeace, said the outcome for Baltimore a decade ago could have been far worse if the cars that derailed had been carrying chlorine. And even though the region avoided casualties, he said, it was hardly cost-free.

"Baltimore shut down for a week and it wasn't even a fatal event," Hind said. "It was a pretty big economic catastrophe for Baltimore."

The event was serious enough that it prompted Congress to commission studies of a possible alternative to continued use of the Howard Street Tunnel and other aging railroad tunnels in the city, a notorious bottleneck for both freight and passenger service.

Advocates for a new tunnel are driven at least as much by capacity issues as safety concerns.

The single-track tunnel slows commerce even when there are no interruptions along the line. And when it was dug in the 1890s, it was built to accommodate trains of that era — not the double-stacked container cars that dominate modern freight traffic. That inability to handle double-stacking on a main access route represents a serious impediment to the port of Baltimore because its chief rival in Norfolk, Va., isn't hobbled by such limits.

A report issued by the Federal Railroad Administration this year, which followed up on a 2005 study, found that "although convoluted and antiquated, Baltimore's railroads have strategic importance far beyond the confines of their immediate region." It labeled the proposal to replace the Howard Street Tunnel "a project of national significance" — a positive signal in the competition for federal funding.

However, the report determined that the freight tunnel project can't match the urgent need to replace the deteriorating, 140-year-old B&P Tunnel on the Amtrak line leading into Penn Station. That $773 million project, the railroad administration said, needs to be completed in the next 10 to 20 years.

The Howard Street Tunnel, by contrast, has no structural issues, the report said. The main justifications found for its replacement were the height issue and the need to relieve congestion on the only rail freight route in the Interstate 95 corridor before growing demand makes it even more of a bottleneck.

"The freight capacity of the Baltimore region is insufficient to handle the expected freight volumes forecasted for 2050," the report said, taking a long-term view typical of transportation studies.

After considering several alternatives, including a pair of harbor crossings, the report recommended a new, double-tracked, roughly $1.15 billion tunnel that would loop under West Baltimore, cross the Jones Falls Valley and link up with the existing Belt Line through Charles Village to the Bayview Yard. Taking an optimistic view, it said such a project could probably begin between 2015 and 2020.

Before such a program could get off the ground, however, Congress would have to craft an agreement to fund it — an effort that would likely require a mix of federal, state and private financing.

Though CSX would appear to be a main beneficiary of a new tunnel, it has been coy about the expensive project. The report notes that the railroad "expressed satisfaction" with the current tunnel and "did not convey interest" in the new freight tunnel project.

Robert Sullivan, a CSX spokesman, confirmed that stance and said the existing tunnel is in good condition.

"It is safe. It is well-maintained. It is frequently inspected both visually and by computer-based equipment," he said.

A few weeks after the 2001 fire was extinguished and the burnt-out cars were dragged away, Bentley wrote an article for The Sun in which she called for the tunnel to be replaced — a move she had been advocating for decades. "The problems simply will not disappear; they will only worsen, and real tragedies involving loss of life are not difficult to anticipate," she wrote.

Bentley, the perennial champion of the port that bears her name, said the inadequacies of the Howard Street Tunnel became apparent as early as 1919.

"Nothing's been done in 90 years, and it's going to be 100 years very soon," she said last week. "Let's celebrate the 100th anniversary by doing something about it."