Being basically lazy here. Coming home from my wife's family's place today I noticed a rail line that went under I-80. Area was a few miles east of the Newton exit. Been through here dozens of times and never noticed it. There was a fairly modern signal a few hundred feet from the I-80 bridge. Like I said, being lazy here, too much turkey, and I don't feel like going through my books to figure out the line. Anyone care to help me out?
I'd suspect you saw the NJT (DL&W) Boonton Line passing under I-80 in Roxbury, near the Lake Hopatcong exit. The Lake station is about a mile north of the double tracks. The US 206N exit for Newton is about three miles farther west.
The other, less likely choice, is at the US 46E on-ramp, and US 206N off-ramp. The Boonton line (single track) passes underneath as it approaches the Mount Olive station in the township of the same name.