The Great Wall of Canton
Built in 1835 for the Boston & Providence Railroad, the Canton Viaduct spans the Neponset River, and is still part of the main line for Amtrak, including the high-speed Acela trains. It has been designated a National Historic Civil Engineering Landmark and is also on the National Register of Historic Places. These days we can't even build bridges that last 50 years.
It was also once part of the Old Colony Railroad network and later the NY, NH&H.