Like many things, im sure the pronunciation was determined by the locals..
considering it hasnt officially existed in over 70 years (absorbed into the Erie in 1941)
we would need to find some 90 year old railroaders to know for sure..
Official name: New York, Pennsylvania and Ohio Railroad.
nickname: NYPANO for "NY, P and O"
Either "Nye-Pa-NO", as Charlie said,
or I would guess "Nip-an-oh" as being likely..
or,the shortest and quickest version to say: "Nip-uh-no"