Frostburg, Md. — THREE hours by bike from Cumberland, Md., on the Great Allegheny Passage, I rode into the mouth of the abandoned Borden Tunnel near here, where freight trains once rumbled, hauling coke, steel and coal. The air was black as engine oil.

In the middle of the tunnel, darkness swallowed my pedals and handlebars. My wheels wobbled on crushed limestone, but I couldn’t see them anymore. My friend David Howard was pedaling far in front of me and loving every minute of it. “I can’t see a thing,” he said gleefully. “It feels like an out-of-body experience.”

But I was so unnerved that my bike spun out of control and I nearly slammed into the wall.

As I discovered on a three-day trip this year, the passage, which travels 132 miles from McKeesport, Pa., to Cumberland, Md., is part industrial history lesson, part nature excursion and part fun house, with thrilling and spooky moments: barely lighted corridors through mountainsides, whitecaps on rivers a hundred feet below and the lonely sound of a freight-train whistle.

Word is getting out that the trail is a world-class biking destination. Linda Boxx, president of the Allegheny Trail Alliance, a coalition of seven organizations that oversee the project, said 10,000 to 15,000 people rode a long-distance trip along it last year. The trail was built at the cost of $65 million after the rail tracks were abandoned in 1975.