who designed it?
James Corner Field Operations, Diller Scofidio + Renfro, and Piet Oudolf.
High Line
The High Line's design is a collaboration between James Corner Field Operations, Diller Scofidio + Renfro, and Piet Oudolf. 
where does the highline in new york city start Gansevoort Street or 34th Street. what is this line? It is a elevated linear park, greenway and rail trail.
wiki:23892687
High Line [SEP] Introduction
The High Line is a elevated linear park, greenway and rail trail created on a former New York Central Railroad spur on the west side of Manhattan in New York City. The High Line's design is a collaboration between James Corner Field Operations, Diller Scofidio + Renfro, and Piet Oudolf. The abandoned spur has been redesigned as a "living system" drawing from multiple disciplines which include landscape architecture, urban design, and ecology. Since opening in 2009, the High Line has become an icon of contemporary landscape architecture. The park is built on a disused, southern viaduct section of the New York Central Railroad's West Side Line.