How it started
Origins
The Central Railroad of New Jersey (CNJ) had reached Phillipsburg, New Jersey in 1852, on the outskirts of the Lehigh Valley, and anticipated extension to Allentown. If a direct route could be built from Allentown to the Susquehanna River, western traffic could reach New York City faster than by detouring through Philadelphia and traveling north over the Camden and Amboy Railroad.
Such a route would run parallel to South Mountain and other mountains in the region, and a lateral railroad for the transport of anthracite coal already extended west from the Susquehanna. The Dauphin and Susquehanna Coal Company (D&S) operated a railroad which connected with the PRR at Rockville (where their main line crossed the Susquehanna), ran north along the river to Dauphin, and turned northeast to run up Stony Valley to the Rausch Gap coal mines. It proved to be a willing partner in the plan.
From Wikipedia’s article on Allentown Railroad, shared under CC BY-SA 4.0.2