Getting outside for some exercise in the Susquehanna Valley can also serve as a history lesson, and you might have passed under a historic structure without ever realizing it.Connecting more than 20 miles from the Mason-Dixon line to the city of York, Heritage Rail Trail County Park is a straight shot through history.It follows a rail line that was a critical part of America's development in the 1800s.But the bikers and runners on the trail today might not notice the antiques around them.There are a lot of places to see along the trail, but halfway between the Brillhart Station and Gladfelter Station is a site that has carved out a place in history: Howard Tunnel.According to York County, Howard Tunnel the oldest continuously operating railroad tunnel in the country.Not only that, it's also one of the oldest tunnels like it in the world. It was dug by hand drills and black powder to get through 300 feet of solid rock, then lined with brick.Howard Tunnel was finished in 1840. During the Civil War, soldiers and supplies funneled through it on their way south.In the days leading up to the Battle of Gettysburg, Union soldiers set out to defend the tunnel. They dug rifle pits below and hauled a cannon to the top. The Confederates eventually came through and burned many of the railroad bridges along the line but didn't damage the tunnel.After the war, the tunnel was rebuilt to accommodate two rail tracks underneath it.When you visit, look for the plaque that designates the tunnel as being on the National Register of Historic Places – one of several such sites on the trail.A train still runs through Howard Tunnel today. The Northern Central Railway is a Civil War replica excursion train.