CSX crews began uncoupling and removing train cars Monday evening from the Howard Street Tunnel in Baltimore, starting the process of clearing a derailment that shut down freight traffic through the city earlier in the day.

The Cumberland-bound train was carrying a volatile, flammable chemical when 13 cars went off the rails Monday morning, but authorities said there were no reports of leaks or injuries.

Work to clear the tunnel was expected to take more than 24 hours.

“This is going to be a long operation,” said Bob Maloney, the city’s emergency management director. “The Fire Department identified there was not an immediate threat to the public. We still consider that to be the case. We’re prepared if that changes.”

The 124-car train went off the rails near the tunnel’s north entrance at the Mount Royal Station in Bolton Hill about 5:45 a.m. Monday, authorities said. But they waited until after the evening rush hour to begin clearing the tracks.

In the event of a chemical spill during the clearing of the derailed cars, authorities said, the Fire Department would use a reverse 911 system to tell residents who live within a quarter-mile radius of the incident to shelter in place, officials said.

“Our meters show there’s no immediate danger,” Assistant Fire Chief Mark Wagner said.

Authorites are investigating the cause of the derailment. It started about one-third of the way through the train at car 47, one of the 18 that were carrying loads, authorities said.

The front of the train had entered the tunnel when the cars derailed just north of the tunnel, Maloney said. The derailed cars continued into the tunnel, where they stopped, he said.

The Philadelphia-to-Cumberland run “is a regular, routine route for this train,” said Brian Hammock, resident vice president of CSX.

Hammock said he did not know when the tunnel was last inspected. He said CSX has full confidence in all of its tracks throughout the city.

A day after the deadliest mass shooting in U.S. history Sunday at a gay nightclub in Orlando, Fla., Wagner called the FBI to help investigate the derailment. “With everything going on, especially in Orlando, I asked the FBI to be here because we want to rule out foul play,” Wagner said.

Investigators determined it was not caused intentionally.

Baltimore Police Commissioner Kevin Davis said his department, too, was assisting. “We want to be on the ground at the very, very beginning in case a twist or turn occurs,” Davis said. “Twists and turns have not occurred, but we’re nonetheless involved right now in this critical incident.”

Several roads were closed near the tunnel Monday. They included a stretch of Howard Street between North Avenue and John Street.

The Maryland Transit Administration announced it was suspending light rail service between the Camden Yards and North Avenue stations after 10 p.m. Monday, and would use buses to ferry passengers between the two stops until midnight.

Freight rail traffic was stopped in the area Monday. The line running through the tunnel is used only by CSX freight trains; Amtrak and MARC service was not affected.

The Howard Street Tunnel is considered to be the most troublesome bottleneck for north-south freight train traffic on the East Coast.

For many years, transportation planners have discussed replacing the tunnel, but the estimated cost — $1 billion to $3 billion — has stymied progress.

In April, the Hogan administration and CSX announced a stripped-down, $425 million plan to expand the tunnel so that double-stacked trains could pass through.The state and the railroad pledged to kick in $270 million for the project and applied for a $155 million federal grant through the U.S. Department of Transportation’s FASTLANE program.

Matthew A. Clark, a spokesman for Gov. Larry Hogan, said the state is waiting for a decision on its application. Federal officials are expected to announce awards this summer.

Since the spectacular tunnel derailment and fire of 2001 halted freight traffic in the corridor for almost a week, there have been a series of smaller-scale incidents along the approaches to the tunnels.

In 2005, a three-car derailment near the site of the 2001 incident prompted then-Mayor Martin O’Malley to call for a federal inspection.

Two years later, 12 cars derailed near M&T Bank Stadium. The next month, a CSX tanker left the rails in Locust Point.

Deadlier CSX derailments have occurred elsewhere in Maryland. In August 2012, two young women who were on railroad property in Ellicott City were killed when a train went off the tracks and spilled a load of coal on them. In 2000, a train left the tracks in the Western Maryland town of Bloomington, crashed into a home and killed a 15-year-old boy.

The last major CSX derailment in Maryland took place in May 2014, when three locomotives and 11 cars left the tracks while crossing a culvert blocked by debris in Prince George’s County. There were no injuries, but the mishap caused more than $300,000 in damage, federal records show.

Environmental advocates and city residents have long voiced concern about freight trains carrying hazardous chemicals through and underneath Baltimore’s neighborhoods. The City Council held a two-hour public hearing last summer on the safety of shipping crude oil through Baltimore.

Keisha Allen, president of the Westport Neighborhood Association, said her home is within a block of freight tracks — well within the “blast zone,” should a derailment cause an explosion.

“That’s the issue, the fact that it’s highly flammable,” she said.

Allen said she and her neighbors want the city to require CSX and Norfolk Southern to disclose what’s being shipped on the freight trains and when.

“There needs to be a clear indication of what’s coming through,” she said. “If it’s something that flammable, that volatile, there needs to be notification, at a minimum. … We would sleep better knowing there’s a process.”

Lawrence Mann, a Washington attorney who specializes in railroad liability cases, said the industry has generally been lax about track inspections.

“The railroads have either fired or furloughed thousands of track inspectors around the country,” he said. “They just don’t have the manpower to do the job that’s required.”

The country’s major railroads spent $28 billion on capital expenditures and maintenance in 2014, the Association of American Railroads reported Monday.

That investment increased to $30 billion last year and is expected to hover around $26 billion this year, said Edward Hamberger, president and CEO of the trade industry group.

That has increased from the roughly $20 billion in annual infrastructure investment between 1983 and 2011, as carriers work to keep up with customer demands for reliability and service, including new double-stack containers, he said.

The investments have also improved safety, Hamberger said. The association recently reported a 79 percent decline in train accidents since 1980.

“A well-maintained railroad is a safer railroad,” he said. “The fact that we can spend this amount of money to put in new tracks, all-new technologies, and maintain it is really a point that needs to be driven home.”

He said some carriers have been reluctant to participate in public-private partnerships because public money typically comes with constraints and major projects often get bogged down in lengthy public permitting procedures.

“It’s never fast enough, but we’re trying to do the best we can,” he said.

Baltimore Sun reporter Natalie Sherman contributed to this article.