Horseshoe Curve is a railroad curve measuring 2375 feet from the beginning to the end of its curvature, with its lower end 1594 feet above sea level and its upper end 1716 feet, a difference of 122 feet. The diameter of the half-circle formed by the Curve is 1300 feet, its degree of curvature is 9 degrees, 25 minutes, and its average grade is 91 feet per mile, or 1.8 percent, with the grade reduced slightly at the center to compensate for the increased friction of the curved track. The Curve opened on February 15, 1854, had two tracks in operation by the end of that year, gained a third track in 1898 and a fourth in late 1899 to early 1900, and lost the second from inside track in 1981 because of declining traffic. It has remained in continuous use with few exceptions, including brief strike stoppages and weather-related closures from the Johnstown floods of 1889 and 1977. Average daily traffic is 60 trains in both directions, not counting returning helpers, compared with 168 trains per day in 1904. Traffic on the Curve includes freight and passenger equipment such as General Electric P42DC locomotives, Norfolk Southern SD40-Es that often operate in pairs and on both ends of long freight trains going upgrade, intermodal traffic descended from truck trailers first carried on flat cars in 1927 and later doublestack trailers and containers, Amtrak Amfleet passenger cars introduced in the mid-1970s, coal hopper trains that are commonly seen with empty cars going upgrade and loaded cars going down, and oversized loads that can sometimes be handled on depressed center or semi-well flat cars despite tunnel clearances at Gallitzin.