About the series: This part of the state has some of Connecticut's biggest and most popular tourist attractions. But in this occasional series we're going to bring you places worthy of a visit that you might otherwise have never heard of.

It's hard to imagine a massive infrastructure project — blasting through rocky hilltops and filling deep valleys — carried out just to create a recreation trail. But thanks to a century of railroad boom and bust, eastern Connecticut boasts a 40-mile path through the forest that is flat and wide enough for hikers and cyclists of all levels to share, and long enough to allow extended stretches of quiet solitude.

Today, the main rail route between New York and Boston follows a relatively flat, if indirect, path along the coast. In the middle of the 19th century, companies in the booming railroad industry sought to establish a more direct inland route between the cities. In his book "Lost Railroads of New England," Ronald Dale Karr describes the difficulties faced by railroads trying to expand through the rugged, rolling terrain of the Connecticut countryside. The steep grades of the state's many river valleys had to be re-engineered through blasting and filling, or spanned by the construction of massive bridges and viaducts.

The Air Line, named for the imaginary straight line drawn through the air between Boston and New York, ran from New Haven to Willimantic. The white-painted express train, known as the Ghost Train, could make the run between Boston and New York in six hours, a record time for 1884. Passenger service on the Air Line ceased by 1937, and the track was completely abandoned by 1965. The former railroad bed is now the Air Line State Park Trail, with a gravel path that stretches almost continuously for more than 40 miles from Portland to Thompson.

The following details are based on a bike ride on the 20 miles of the southern section of the trail from East Hampton to Willimantic. If you're not up for pedaling as far, numerous shorter excursions are possible from the 16 parking areas along the trail. Mileage is measured from the trailhead on Main Street in East Hampton.

Mile 3: Lyman Viaduct

A brief paved section of the trail climbs uphill from Main Street before turning to gravel and passing a wetland known as the Cranberry Bog. Brian Holdt, owner of AirLine Cycles, a bike shop located a few hundred feet from the trailhead, says the first three miles have the best scenery on the entire trail.

The sheer amount of work that went into laying the track back in the 1860s and 1870 is apparent as you ride through deep cuts with tall rock walls on either side. Some are narrow enough that you could imagine a train car passing with only a foot or two to spare. Between the cuts are massive man-made ridges that allowed the railroad to cross the river valleys at grade. Two of these fills, the Rapallo Viaduct in East Hampton and the Lyman Viaduct in Colchester, were once towering bridges before they were filled with sand and gravel to accommodate heavier freight trains.

Historical photos of the Lyman Viaduct show trains on a track atop a metal trestle 137 feet above Dickinson Creek, spanning 1,000 feet from one side of the valley to the other. The modern experience is not as immediately impressive until you look to your right and realize that your wheels are at a higher elevation than the treetops growing from the valley floor. One mile farther to the northeast, the embankment to the right falls away even more steeply down to the Salmon River, which eventually crosses under a modern wooden bridge affixed to the original stone abutments left over from the railroad era.

Mile 10: Raymond Brook Marsh

There are stretches of solitude on the trail as it runs flat and straight through the forest, but even on a dreary Wednesday in May, dozens of runners, dog walkers and cyclists are on the trail. The straight, flat stretches make it hard to perceive whether you are climbing or descending. Often the only clue is how hard you have to pedal to maintain a constant speed.

Go too fast, though, and you might miss something that the surroundings have to offer.

After a rainy spring, wide ponds border the trail on both sides as one rides through the Ramond Brook Marsh in Hebron. Despite the clatter of pedals and the wind rushing past, you can still hear the tweets and trills of songbirds flitting overhead. But had I not stopped to take a picture of a small snapping turtle basking on the gravel trail, I never would have heard the low-pitched chorus of frogs that emanated from the surrounding wetlands.

Miles 11-16: Old railroad stations

After leaving the marshlands, you pass a spur trail leading south to Colchester and come upon a large mural on the back side of Juliano's Pools in the Amston section of Hebron. The painting depicts people in period dress waving to an old-fashioned engine pulling into a railroad station.

While the original Air Line was meant for express trains between New York and Boston, there were local stations along the route. A trail map and brochure for the trail's south section shows that stations were located at the crossings at Leonard Bridge Road and Route 87 in Lebanon. The trailhead at 87 has old photographs of the stations that were used by local farmers and schoolchildren.

Miles 17-20: Farmland, Willimantic and beyond

With the exception of a short patch of road crossing under Route 2 in Colchester, the trail is almost always surrounded by forests and swamps. You are never far from a road crossing, though, and plenty of benches and picnic tables are situated along the way. Just northeast of the 87 crossing in Lebanon, a small wooden pavilion with an octagonal picnic table is perfect for taking a lunch break.

As the trail winds toward Willimantic, it passes a small farm where the cows graze just a few feet from the trail, seemingly disinterested in any cyclists that pass by.

A few miles later, a modern wood and concrete bridge spans the Willimantic River, and a paved trail brings you past the junction with the Hop River State Park Trail, another rail trail that stretches northwest to Manchester and Vernon. The Air Line Trail brings you to the entrance of the Connecticut Eastern Railroad Museum and then continues on through downtown Willimantic.

It's another 20 miles back to East Hampton, with a short extension into Portland. But if that's not far enough for you, adventurous cyclists can continue on toward the Massachusetts border in Thompson. According to maps on the DEEP website, connecting the trail through Pomfret, Putnam and Thompson requires the use of roads to bypass unfinished areas of the trail.