The Third Avenue El, featured in this short - officially the IRT Third Avenue Line - was originally built starting in 1878 and was completed in 1920. Demolition began in 1950 and was completed in 1973. Manhattan had three other elevated rail lines running over 2nd, 6th and 9th Avenues. All were closed 1938-42 and demolished.
This short documentary is featured on the Criterion Collection DVD for The Horse's Mouth (1958), released in 2002.
The short can also be found on the 'Bob Dylan: Dont Look Back (1967)' Criterion Collection release from 2015.