Amtrak ridership has exploded between Washington and New York over the past 15 years, with rail service in the Northeast helping to defray the losses of the federally subsidized passenger train service, according to a new study.

Ridership in the Washington region increased by 60 percent in the 15 years from 1997 to 2012, examined in the Brookings Institution study released Friday.

The Washington region had 5.8 million riders in 2012, making up 9.3 percent of Amtrak's ridership.

Only the New York metropolitan area had more riders than the D.C. area, with 10.9 million in 2012.

Joshua Schank, president and CEO of the Eno Center for Transportation, said the Washington-to-Boston line, served by the money-making Acela and other trains, benefited from the lack of adequate alternatives for commuters.

"You've got a very congested highway network that is limited to only one route," he said. "That is not a fun drive, and rail has a very effective means of competition."

The Boston area saw the largest percentage increase in riders, jumping 211 percent from 1997 to 2012 to a total ridership of 3.2 million.

Four highly trafficked Amtrak lines located in the Northeast brought in more money than they spent, while the remaining 40 Amtrak routes lost hundreds of millions of dollars. The four money-making lines ultimately support Amtrak's budget, shouldering some of the cost of unprofitable lines.

"It's a national system. It has one number at the end of the day," said Robert Puentes, one of the authors of the study.

The Acela line, which runs from D.C. to Boston, made the most money. It brought in $178.8 million in fiscal 2011.

Three other lines make money: the Northeast Regional, the Washington-Lynchburg and the Adirondack.

Amtrak's 15 longest routes, running more than 750 miles each, lost $597.6 million in 2011. The three medium-length routes, between 400 and 750 miles, lost $16.3 million altogether. But routes less than 400 miles brought in more revenue than they spent, raking in $46.6 million from 26 routes.

Some key costs are not included in that calculation, such as interest and the cost of the tracks. State funding also counts as revenue because it is not provided by the federal government.

Besides making money, routes less than 400 miles long drew dramatically more riders than longer trips in both 1997 and 2012.

In 2012, 83 percent of riders took trips less than 400 miles, while only 2 percent took trips between 400 and 750 miles. The other 15 percent traveled more than 750 miles.