Rail cars burn at the scene of a freight train derailment in March 2015 near Galena, Ill. (Mike Burley/Telegraph Herald via AP)

After a weekend in which two freight trains derailed, spilling their contents, an advocacy group’s report released Tuesday says bridges that carry dozens of those types of trains each day are in dangerous disrepair.

Although neither weekend derailment involved a bridge, one resulted in the leak of almost 20,000 gallons of ethanol into the Mississippi River. Fear that a train could topple from a decrepit bridge to pollute a river or marshland led the Waterkeeper Alliance to begin inspecting rail bridges.

The report says “citizen inspectors” who visited 250 rail bridges in 15 states found that 114 of them had deteriorated badly. In some cases, the report says, the inspectors were present when oil trains crossed the bridges and “observed flexing, slumping and vibrations that caused concrete to crumble.”

The report calls for stronger federal oversight of the railroads that own an estimated 100,000 bridges in the United States. Congress in 2008 mandated that railroads inspect their bridges annually, subject to review by federal regulators.

“When the railroads conduct bridge inspections and do find safety issues, federal officials do not need to be informed and have very little authority to compel rail bridge owners to make repairs,” Larissa Liebmann, a lawyer with the Waterkeeper Alliance, said in a conference call with reporters.

Sarah Feinberg, the administrator of the Federal Railroad Administration, admonished railroads last week to be more forthcoming about the condition of their bridges.

“When FRA is asked about bridge safety, it’s frequently because the public or a member of Congress becomes concerned and has tried to get answers from a railroad, and they have been ignored or put off,” Feinberg said. “Now, I know that railroads are expending significant resources on maintaining bridges. I know that just because a bridge isn’t pretty doesn’t mean it isn’t in good shape and it isn’t going to remain in good shape for decades to come. But members of Congress come to the railroads to ask these questions; they are coming away unconvinced.”

Feinberg is reviewing the parameters of the 2008 mandate for bridge inspections and may call for an FRA inventory of bridges if Congress provides the resources to conduct one.

A spokesman for the Association of American Railroads said rail bridges were “among the safest segment of the nation’s infrastructure” and cautioned against judging a bridge by its appearance.

“Some bridges are painted, some are not. Some are more weathered that others,” said Ed Greenberg, the spokesman. “Inspectors scrutinize a bridge to assess its structural integrity with no relationship to whether it’s aesthetically pleasing.”

The conference call with the Waterkeeper group made clear its paramount concern: the cargo being carried by the trains.

The boom in domestic oil production in the Bakken oil fields has caused the number of rail tank cars carrying flammable material in the United States to grow from 9,500 seven years ago to 493,126 last year.

“The rails follow the rivers in this part of the country,” said Krissy Kasserman, the Youghiogheny riverkeeper in western Pennsylvania. “Oil trains here pass within a stone’s throw of several drinking-water intakes.”

Those trains roll from the oil fields in North Dakota, Montana and Saskatchewan to refineries on the East, West and Gulf coasts.

On Saturday, a Burlington Northern and Santa Fe Railway locomotive pulling a mix of tank cars and automobile carriers derailed near Alma, Wis. Six tank cars were among the 25 that derailed, and five of them leaked ethanol into the Mississippi.

The next day, 35 families were evacuated in Watertown, Wis., more than 200 miles from the first accident, when 13 tank cars of a Canadian Pacific train derailed. One car leaked oil.

All of the U.S. derailments occurred in remote areas. But in July 2013, a runaway freight train in Canada carrying 74 tank cars full of Bakken oil derailed in the town of Lac-Megantic, Quebec, setting off a fire that destroyed 30 downtown buildings and killed 47 people.

Nine tank-car derailments, including the two over the weekend, have occurred in the United States and Canada this year, with four resulting in explosions or fires.

• Feb. 4: Fourteen tank cars carrying ethanol jump the tracks north of Dubuque, Iowa, and three of them burst into flames.

• Feb. 15: Fire breaks out in a remote wooded area of Ontario after a 100-car train derails.

• Feb. 16: Twenty-eight tank cars carrying crude oil derail and catch fire in rural West Virginia.

• March 5: Twenty-one tank cars derail and leak crude oil within yards of a tributary of the Mississippi River in rural Illinois.

• March 7: Five tank cars tumble from a bridge in Ontario, some of them on fire and some leaking oil into a waterway.

• May 7: A 71-tank car train derails in Heimdal, N.D., and bursts into flames, forcing the evacuation of several dozen people from the unincorporated town.

• July 16: A train hauling oil to a refinery on the coast of Puget Sound derails and spills 35,000 gallons about five miles east of the small town of Culbertson, Mont.