The LIRR spur never ran to downtown Northport.
The trolley from the "new" Northport station ran to downtown Northport.
The spur was the LIRR, not the trolley line (which WAS affiliated with the LIRR, by the way.)
The spur led to what was originally the Northport station before the line extended to Port Jefferson, and later, to Wading River. The station was built between January-March, 1868. The terminal facilities there consisted of a 2-stall engine house, a turntable, watering and coaling facilites and the depot.
When a junction was installed south of the station to continue the tracks on to Port Jeff, the station was named on the timetables of the day "Old Northport" so as not to confuse it with the newer Northport station opened further east.
There was passenger service to BOTH Northports for a very limited time until "Old Northport" was discontinued as a station stop on 10/17/1899.
The spur remained a freight-only line, in its later years servicing Axinn Lumber located at the end of the spur.
A railfan extra utilizing the Budd RDCs ran up the branch around 1956. It was the only "passenger" service since the station was closed in 1899.
The line was removed some years ago per Art Huneke's website and photo evidence.
Dave Keller