The Ronkonkoma Branch is the marketing name given to the core electric service of the westernmost 49 miles of the Main Line of the Long Island Railroad since electrification was extended from Hicksville to Ronkonkoma in 1987, allowing passengers riding this branch to no longer have to transfer at Jamaica and decreasing travel times from Ronkonkoma to Penn Station from 97 minutes to 71 minutes. Service on the line, using up to 12 car long electric trains, operates hourly or better throughout the day between Ronkonkoma and Penn Station. During rush hours there are a few trips to and from Atlantic Terminal, and before COVID-19 related service reductions, one PM peak trip originated at Hunterspoint Avenue (a legacy of the lines pre-1987 diesel only service). This expansion project also built all new stations (in new locations) on the line east of Farmingdale, all with large and soon overcrowded parking lots, with many passengers driving who used to ride trains from closer to their homes stations on the Port Jefferson, Montauk, or now diesel Greenport Branch stations east of Ronkonkoma now driving to Ronkonkoma Branch stations for faster travel times.

The line east of Farmingdale after electrification was originally single-tracked with passing sidings which meant reverse peak service between Farmingdale and Ronkonkoma was non-existent. No Ronkonkoma-bound trains left Farmingdale for Ronkonkoma between 6:04am and 8:35am and no Penn Station-bound trains originated in Ronkonkoma between 4:48pm and 7:13pm. The ROW was wide enough for a second track (with no property acquisitions) and the design/build contracts to complete double-tracking was completed in two phases with hourly or better reverse-peak service beginning on September 21, 2018. This project added 13 miles of new track, supplementing the existing 5 miles of double-track on the line.