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There is a trail etiquette that’s policed by trail users themselves. General rules of the road may apply: “Stay to the right, alert people when you’re passing, those kinds of things,” Ms. Pack said.

Rick Wagner, 63, of Piercetown, Ind., and his family have been biking on rail trails for 10 years. “We just started seeing how many different states we could ride a trail in,” he said. So far, they have hit 14.

“You meet really neat people on the trails,” he said. “It’s just like everyone’s on their best behavior.”

A railbed makes a nice biking trail, he said, because “it’s flat, and most of the trails have some shade from trees on one side or the other.”

He said he rode 60 miles on the Little Miami Scenic Trail in Ohio when the thermometer reached 100 degrees. “I didn’t even know it was that warm,” he said.

Mr. Wagner and others have raised about $220,000 to develop a new trail near his home. They’ve built a trailhead with heated restrooms and lighting. “You need that for people to park and get on the trail,” he said.

The latest addition to the trail movement is water trails, according to Stockman. In February, the Chattahoochee River Water Trail in Georgia became the first to be designated under the new National Water Trails System.

“The right of way is already there,” Mr. Stockman said. All that was needed were access points. “You put your canoe in here and take it out there and get your son to drive your car around to the end,” he said.