The Harlem Line runs 82 miles from Grand Central north to Wassaic through the interior of Westchester, Putnam, and Dutchess Counties. The line is the only Metro-North East of Hudson Line not to share any trackage with Amtrak intercity trains, and the only mainline line to have a true end of the line at Wassaic (with the remainder of the line up to Catham, New York in Columbia County now a rail trail starting in Wassaic after being abandoned in 1972).
Starting at Grand Central, the line is 3rd rail electric the first 53 miles until Southeast (Brewster North) where electric service was extended to from North White Plains in 1984, and diesel only the final 29 miles to Wassaic. The number of tracks also decreases as you get farther north. From Grand Central through Mount Vernon West are a minimum of 4 tracks, there are 3 tracks until Crestwood (the third track was opened in the existing rail ROW in 2004, allowing better reverse-peak service), and two tracks from Scrasdale through Southeast. The diesel line up to Wassaic is a single-track with passing sidings. The line is also fully-grade seperated from Grand Central until just north of North White Plains. The legacy of electrification not happening until 1984 makes the line between North White Plains and Southeast, the only stretch of Metro-North mainline to have grade-crossings.
The line recieved high-level patforms at all stops up to North White Plains for the introduction of M1 cars between 1971 and 1975. The stations continuing north to Southeast all recieved a high-level island platform with a short overpass structure across one track with staircase/elevator access, even though some stops are near grade-crossings, in the early 1980s in time for electrified service in 1984. All stops up to Dover Plains recieved high-level platforms in 1990s, with the 6 mile extension to Wassaic opening on July 9, 2000 with high-level platforms at it's two new stops. The only exceptions to this are the two limited service stops, the cemetary Mt. Pleasant stop which is a tiny concrete high-level platform where one door is keyed open, and the Appalachian Trail stop only designed for hikers (and only rail station directly on the famous 2,190 miles trail) and only stop on the Harlem Line where diesel shuttle trains use the traps and steps built into the Shoreliner doors to open one door.
For service, before the COVID-19 pandemic three trains per hour or better operated in each direction:
Due to the COVID-19 pandemic service was reduced on the Harlem Line to hourly only on March 27, 2020 (ridership dropped 94% with stay at home orders) with one local train per hour making all stops from Grand Central to North White Plains, connecting with an hourly express train in White Plains from Grand Central to Southeast. In peak hours additional trains provide service roughly every 30 minutes. Trough service to Wassaic was discontinued with connecting Shuttle Service every two hours on weekdays only. Weekend diesel service between Southeast and Wassaic was discontinued beginning the Weekend of April 4 and as of November 2020 hasn't been restored. This hourly scedule had previously been used on major holidays like Thanksgiving (after extra-morning service for Macy's Thanksgiving Day paraders), Christmas, and New Years (after New Years Eve was the one night a year that Metro-North didn't stop running overnight).