Pedaling into Taft Tunnel, which burrows beneath the Bitterroot Mountains at the Montana-Idaho border, my sons and I quickly plunged into blackness. Scents of damp rock, the sounds of water running down the walls and an abrupt drop in temperature enveloped us in otherworldliness.

The tunnel at St. Paul Pass, a signature link in the Milwaukee Road's rail route between Chicago and Puget Sound, was as tall and wide as a boxcar. It was, however, 1.6 miles long, and the head lamps strapped onto our bike helmets threw only meager beams onto the cement floor. We'd entered on the Montana side, what's known as the east portal. Several minutes passed before the west portal -- the proverbial light at the end of the tunnel -- came into view.

We emerged in Idaho, and parked next to a waterfall where we warmed up in the sun. It was just 55 degrees in the tunnel. Changing states had been big events for my sons, Patrick, 9, and Colin, 12, on our summer road trip, but this subterranean passage through the Rockies topped everything. We even changed time zones.

The Taft Tunnel was just the beginning of our ride on the Route of the Hiawatha -- a 15-mile bike path passing through 10 tunnels and across seven high trestles. Dropping about 1,000 feet from the pass to its endpoint in Pearson, Idaho, the trail is an easy downhill ride, a fact reinforced by the families, children and seniors on the trail. The hard gravel path and concrete floor of the tunnels are suitable for hybrid bikes, even tagalongs and run-behind baby carriers.

Owned by the U.S. Forest Service, the Hiawatha is one in a series of rails-to-trails spanning the Idaho Panhandle -- including the 72-mile-long, paved Trail of the Coeur d'Alenes.

The ease of rail-to-trail paths

In Idaho and elsewhere, the rails-to-trails movement has bloomed with the demise and consolidation of major railroads. The United States' rail infrastructure shrank from 270,000 miles in 1920 to 120,000 today. Some 19,000 miles nationwide have been converted to biking trails. Unlike the rugged single tracks favored by mountain bikers, rail right-of-ways are rather wide and flat -- typically less than a 2 percent grade because trains have difficulty with hills.

The Milwaukee Road, officially known as the Chicago, Milwaukee, St. Paul and Pacific Railroad, abandoned its Pacific extension in 1980. The name, Route of the Hiawatha, evokes the legacy of its premier long-distance passenger train, the Olympian Hiawatha. Orange and maroon with distinctively styled sleeper observation cars, the Hiawatha was as much admired for its aesthetics as its speed, which topped out at 100 miles per hour.

Much of this history was presented on the trail with interpretive signs placed at pull-offs and rest areas where we stopped to snack and drink prodigiously from our water bottles. It was a hot day, in the 90s, and tinder dry. Douglas fir and lodgepole pine scented the air. Wildflowers bloomed along the trail's edge.

Occasionally, we stopped in a tunnel for the natural air conditioning, but lingered longest out on the sun-drenched trestles, mesmerized by the panorama of mountains and the mossy, emerald creeks running below the supporting ironwork. I sucked in my breath when I dismounted the bike and stepped onto a wooden-plank walkway on either side of the path. Some trestles stand more than 200 feet high, well above the crowns of the tallest trees in the valley. And though several strands of thick wire functioned as a robust guardrail, the cables present little visual barrier to what was a dizzying view.

Bikers can complete the Hiawatha trail in as little as two hours, but with our gawking, picture taking and my examination of the interpretive signs (the boys pleading, "Dad, stop reading everything") we took nearly five hours.

We loaded ourselves and bikes onto a reconverted school bus for a shuttle back up the mountain. Climbing up the switchbacks of the forest roads with steep, bare hillsides falling away 200 or 300 feet, the bus was its own thrill ride.

Turtles, beavers along the trail

We crossed from Montana to Idaho again, this time in our minivan via Interstate 90. We had a late lunch in the historic mining town of Wallace (population 906), also famous for having the last traffic light on the interstate. When the federal and state DOT proposed razing several blocks of Wallace to make way for the highway, residents fought back by placing the entire downtown on the registry of historic places. It wasn't until 1991 that the highway builders completed a viaduct over the town. The structure isn't architecturally compatible with the town's 19th- and early 20th-century brick buildings, but it spared gems like the Northern Pacific Railroad Depot, now a museum. We spent a couple of hours there exploring the golden age of railroading, looking at photographs of the Taft Tunnel construction and touching artifacts, including a very loud locomotive bell that the curator urged the boys to ring again and again.

The next day, while biking the Trail of the Coeur d'Alenes, we crossed the 3,000-foot-long Chatcolet Bridge, built in 1921 as a swing bridge, which once carried trains over the southern end of Lake Coeur d'Alene. Modified into a bike and pedestrian bridge with a middle arch allowing clearance for boats, the bridge offered a wonderful vista of blue water and low, forested mountains.

Our destination was the village of Harrison. Running up the east shore, the trail skirted the water's edge. Beavers, muskrats and turtles scurried away at our approach. Blue herons stared from the shallows and a bald eagle flew low overhead.

In Harrison, we took a dip at the swimming beach and then walked into the village for french fries and ice cream. We rode back to camp in the long light of late afternoon, our shadows pedaling along beside us.

The rails-to-trails advocates in Idaho and Montana hope to tie together another hundred miles of abandoned right of way and forest roads to form a circuitous ride, what they are calling the Bitterroot 300K Loop. Meanwhile, Montana is extending the Hiawatha trail 31 miles to the east.

All good moves that make the region a mecca for bikers, but as I passed back over the Chatcolet Bridge, I had twinges of longing. In my mind's eye, I conjured up sleek trains -- the so-called streamliners -- running on this right of way, passengers gazing out from dining cars or sipping cocktails in the Hiawatha's Skytop lounge under huge expanses of glass. The Bitterroots and the Idaho Panhandle featured some of the prettiest train rides in America. I would have liked to see the country that way, too.

James McCommons is the author of "Waiting on a Train: The Embattled Future of Passenger Rail Service, a Year Spent Riding Across America." He lives in Marquette, Mich.