The construction of the Lackawanna Cut-Off, a 28.45-mile (45.79 km) railroad line that shortened a key route for the Delaware, Lackawanna, and Western Railroad, took place in New Jersey from 1905 to 1911.

After running for several decades, the Lackawanna Cut-Off became inactive, but in this century, the Lackawanna Cut-Off Restoration Project has brought parts of it back into operation.

During 1905–06, 14 routes were surveyed (labeled with letters of the alphabet), including several that would have required long tunnels. On September 1, 1906, a route without tunnels was chosen. This New Road (Route "M") would run from the crest of the watershed at Lake Hopatcong at Port Morris Junction to 2 mi (3.2 km) south of the Delaware Water Gap on the Pennsylvania side of the Delaware River at Slateford Junction.

At 28.45 miles (45.9 km), the line would be about 11 mi (18 km) shorter than the 39.6-mile (63.7 km) Old Road. The new route would have only 15 curves – 42 fewer curves than the Old Road, the equivalent of more than four complete circles of curvature – which increased speeds and decreased running time – more so for freight, but for passenger trains as well. The ruling grade was cut in half from 1.1% to 0.55%. The new line would also be built without railroad crossings to avoid collisions with automobiles and horse-drawn vehicles.

DL&W chief engineer George G. Ray oversaw the project. Given the size and remote location of the project, Assistant Chief Engineer F.L. Wheaton was assigned the task of overseeing the construction in person.

The project was divided into seven sections, one for each contracting company. Sections 3–6 were 5 miles (8 km) each; Sections 1–2 and 7 were of varying lengths. (Theoretically, to divide the 28.45-mile (46 km) line evenly, the seven sections should have been just over four miles each, but that would have placed the Pequest Fill entirely within Section 3 and the two viaducts within Section 7.) The amount of work per mile varied; the largest share apparently went to David W. Flickwir, whose Section 3 included Roseville Tunnel and the eastern half of the Pequest Fill. (During construction, Lackawanna Railroad Chief Engineer Lincoln Bush would leave the railroad and join Flickwir's construction company, which would take the name Flickwir & Bush.)

Uncertain national economic conditions in 1907 delayed the official start of construction until August 1, 1908.

To accommodate the labor gangs, deserted farmhouses were converted to barracks, with tent camps providing additional shelter. These workers, many of whom came from Italy and other foreign countries or other parts of the U.S., were recruited and would move on to other projects after their work on the Cut-Off was completed. These workers were viewed with suspicion by the local populace in Warren and Sussex counties, with the town of Blairstown going as far as hiring a watchman at $40 per month for the duration of the project. Supervisory personnel and skilled laborers stayed in local hotels, boarding houses, or local farmhouses, usually at exorbitant rates ($1–2 per day) during the years of construction.