 The line was originally proposed in 1919 as part of a massive expansion of what would become the Independent Subway System ( IND ) . Work on the line never commenced , as the Great Depression crushed the economy . Numerous plans for the Second Avenue Subway appeared throughout the 20th century , but these were usually deferred due to lack of funds . In anticipation of the never @-@ built new subway line , the Second and Third Avenue elevated lines were demolished in 1942 and 1955 , respectively . This left the Lexington Avenue Subway as the only rapid transit line on much of Manhattan 's east side ; today , it is by far the busiest subway line in the United States with an estimated 1 @.@ 3 million daily riders . 
