The modern Washington Secondary incorporates the western end of the former main line of the Morris and Essex Railroad.

[1] The Delaware, Lackawanna and Western Railroad leased the Morris and Essex on January 1, 1869.

[4] In 1994, NJ Transit extended Boonton Line service from Netcong to Hackettstown.

NJ Transit leased that portion of the Washington Secondary from Conrail at that time.

NJ Transit acquired the Hackettstown–Netcong segment from Norfolk Southern in 2023, truncating the Washington Secondary to Phillipsburg–Hackettstown.