which year was it opened to the public?
February 15th, 1854.
Horseshoe Curve (Pennsylvania)
The entire line, including Horseshoe Curve, opened on February 15, 1854. 
what is the horseshoe curve in pennsylvania? It is a three-track railroad curve on Norfolk Southern Railway's Pittsburgh Line in Blair County. what is the total length of it? UNANSWERABLE who designed it? UNANSWERABLE which company built this? The Pennsylvania Railroad.
wiki:13026457
Horseshoe Curve (Pennsylvania) [SEP] Construction
As demand for train travel increased, a third track was added to the curve in 1898 and a fourth was added two years later. From around the 1860s to just before World War II passengers could ride to Kittanning Point station near the Curve. Two branch railroads connected to the main line at Horseshoe Curve in the early 20th century; the Kittanning Run Railroad and the railroad owned by the Glen White Coal and Lumber Company followed their respective creeks to nearby coal mines. The Pennsylvania Railroad delivered empty hopper cars to the Kittanning Point station which the two railroads returned loaded with coal.