The Sixth Avenue Line, constructed in stages during the 1930s, was the last trunk line built by the Independent Subway System (IND) before it was incorporated into the modern-day New York City Subway. It was more difficult to build than other subway trunk lines in New York City because construction had to proceed around, over, and under existing tunnels and elevated structures. The Sixth Avenue Line replaced the Interborough Rapid Transit Company (IRT)'s Sixth Avenue elevated, which closed in 1939. The first section of the line opened in 1936 from West Fourth Street to East Broadway with service provided by Eighth Avenue Line trains. This section was initially referred to as the Houston-Essex Street Route. The Sixth Avenue subway was completed in 1940, providing service north of West Fourth Street, connecting to the Queens Boulevard Line and the Eighth Avenue Line.