Original: The first underground line of the subway opened on October 27, 1904, almost 35 years after the opening of the first elevated line in New York City, which became the IRT Ninth Avenue Line.
Simplified: The IRT main line, which is considered to be the first New York City "subway" line, opened in 1904; however, the Ninth Avenue Line, a predecessor elevated railroad line, operated its first trial run on July 3, 1868.