The Main Line is a rail line owned and operated by the Long Island Rail Road in the U.S. state of New York. It begins as a two-track line at Long Island City station in Long Island City, Queens, and runs along the middle of Long Island about 95 miles (153 km) to Greenport station in Greenport, Suffolk County. At Harold Interlocking approximately one mile east of Long Island City, the tracks from the East River Tunnels and 63rd Street Tunnel into Manhattan intersect with the Main Line, which most trains use rather than using the Long Island City station.

Continuing east, five branches split from the Main Line. In order from west to east, they are:

West of Ronkonkoma station, the Main Line is largely double tracked and electrified with 750V DC third rail, with trains governed by Automatic Block and Interlocking Signals and by Automatic Train Control. The line contains a third track between Divide and Queens Interlockings and a fourth track between Queens and Harold Interlockings. East of Ronkonkoma to Greenport, the line is not electrified and trains operate in non-signaled dark territory, with all train movements being governed by timetable and train order authority. Passenger service east of Hicksville station is covered by Ronkonkoma Branch timetables, as it is the final connecting point to other services.

The Main Line has one track from just east of Long Island City, where it splits into two tracks just before Borden Avenue, which continue through Hunterspoint Avenue station to Harold Interlocking (HAROLD, 0.6 miles (0.97 km) northwest of the Woodside station), where the four track Northeast Corridor from Penn Station in Manhattan joins the Main Line after passing through the East River Tunnels. East of HAROLD, the four-track Main Line runs adjacent to the two-track Port Washington Branch until, 0.7 miles (1.1 km) southeast of the Woodside station, the Port Washington Branch turns northeastward. The Main Line continues southeast with four tracks to JAY Interlocking where it meets the Atlantic Branch and Montauk Branch at the west end of Jamaica station. Eight platform tracks and two bypass tracks pass Jamaica station, along with a few yard tracks and two former freight tracks on the south side that can be used by trains bypassing Jamaica. At HALL Interlocking just east of the station there are eight through tracks: two usually westward tracks for Main Line and Montauk trains, two Atlantic Branch tracks that are about to duck under and turn southeast, two usually eastward Main Line/Montauk tracks, and the two former freight tracks on the south side of Hall tower.

Just east of there, Montauk Branch trains get their own two tracks in the center of the four Main Line tracks until the Montauk tracks fly over the other tracks and head southeast. At QUEENS Interlocking, just inside Nassau County between the Queens Village and Bellerose stations, the four-track Main Line splits into the three-track Main Line and the two-track Hempstead Branch (with one track shared by both lines); the four tracks continue parallel to Floral Park station, after which the Hempstead Branch curves away southward and the three-track Main Line continues east to Mineola. East of Mineola, the Oyster Bay Branch splits from the northernmost Main Line track and curves to the north. The Main Line then continues east from Mineola to Hicksville, where the two track Port Jefferson Branch begins and curves to the north. At Hicksville, the Main Line reverts to two tracks. From FARM Interlocking (just east of Farmingdale station), the Main Line continues to Ronkonkoma, except for some freight sidings along the route.

The Main Line west of Jamaica to Harold Interlocking is the only line that connects to the East River Tunnels and the 63rd Street Tunnel, so it is used by all trains operating to New York Penn Station and Grand Central Madison, both in Manhattan. The portion between HAROLD and the Long Island City station is used by trains originating or terminating at Hunterspoint Avenue or Long Island City.

East of Jamaica station, the Main Line is used by all trains on the Hempstead Branch (diverging east of Queens Village), the Oyster Bay Branch (diverging east of Mineola), the Port Jefferson Branch (diverging east of Hicksville), and the Ronkonkoma Branch (terminating at Ronkonkoma, the eastern limits of the line's electrification). Some Montauk Branch trains use the Main Line on their way to Babylon via the Central Branch, diverging east of Bethpage.

Only a few diesel shuttle trains, informally known as scoots, operate between Ronkonkoma and Greenport.