Construction commenced in 1872 as soon the Easton and Amboy was formed; coal docks at Perth Amboy were soon constructed, and most of the line's new expanded New Jersey route from Easton to Perth Amboy was graded and rails laid. However, the route required building a wooden bridge over the Delaware River to connect Easton with Phillipsburg, New Jersey and build a tunnel through/under Musconetcong Mountain near Pattenburg, New Jersey (about twelve miles east of Phillipsburg), and that proved troublesome. The tunnel delayed the opening of the Easton and Amboy part of the line until May 1875 (the line was completed in 1875), when a coal train first passed over the line. To support the expected increase in traffic, the wooden bridge over the Delaware River at Easton was also replaced by a double-tracked, iron bridge.