Originally constructed in the mid-19th century as a New York and Harlem Railroad route, the Park Avenue main line was initially a street railroad and ran to what is now Lower Manhattan. It was gradually truncated through the 1860s, until Grand Central Depot was opened at 42nd Street in 1871. The line was placed in a grade-separated structure in the late 19th century as part of the Fourth Avenue and Park Avenue Improvement projects, and was electrified in the first decade of the 20th century as part of the construction of Grand Central Terminal. Since then, several improvement and rehabilitation projects have been made along the main line.