“Think like a train,” said the man in the Lookout Pass bike-rental shop, handing me a map.

I was about 60 miles east of Coeur d’Alene, in the Bitterroot Mountains on the Idaho-Montana line. My rented bike and rack in place, I drove up an unpaved mountain road to the mouth of a 1909 railway tunnel.

It looked dark in there.

For decades, this was the route of the Olympian and Olympian Hiawatha passenger trains that ran between Chicago and Seattle-Tacoma. The trains stopped operating about 1980. In the late 1990s, workers hauled away nearly 15 miles of track. Just like that, a mountain-bike trail was born.

OK. Into the tunnel. I had been warned that spring runoff would be dripping from the walls and ceiling, and that it would be really dark, which was why my bike was outfitted with a powerful headlight. Also, they had told me, there would be the occasional pothole. The temperature would drop to 42 degrees.

The tunnel, officially known as St. Paul Pass, is 1.7 miles long, starting in Montana and ending in Idaho.

It’s a shame one can’t bottle such an experience.

In the tunnel, you can lose track of time and space so that you feel suspended in near-blackness, water splashing, your legs pumping away, a faint light hovering like a distant keyhole. Then the daylight bursts upon you and you’re gliding out under the big sky, a green valley unfolding with trickling creeks and chirping birds.

From there, the route bends and descends. In the next 9.4 gentle downhill miles, I pedaled across low and high trestles, their metal supports rising from the forest floor like misplaced Erector Sets. And I passed through several shorter tunnels, some curved.

Then, grateful that the railroad engineers had designed the grade at less than 2 percent, I turned and pedaled back the way I’d come. (In summer, a daily shuttle bus carries weary riders back to the top.)