Grand Central Terminal to Mott Haven Junction

The present Grand Central Terminal was built between 1908 and 1913, on the north side of 42nd Street at Park Avenue. The famous great hall, with gates to the express train tracks, is located one level up from the surrounding street, and is served by ramps from street level. Beneath the great hall is the suburban concourse, replete with eating establishments as well as gates to the suburban track level. There are two complete levels of tracks at GCT, with the lower-level suburban tracks having eight full-length stub platform tracks (Tracks 105-112), with two shorter stub-end tracks on the west side (113 and 114) and one shorter stub-end track on the east side (104) There are three more platform tracks on each side (101 to 104 on the east side, and 115 to 117 on the west side), connected through on the reversing loop behind the stub-ends and the concourse. While all tracks remain in use, the reversing loop is almost never used in the 21st century.

At the Express Train level, there are eighteen full-length stub-end tracks (Tracks 16-33), with four shorter stub-end tracks (34-37) on the west side, and two stub-end tracks (14 and 15) formerly used for mail trains on the east side. Tracks 4 to 13 on the east side were once used for "express", and as on the lower level, there are three tracks on the east side (1 to 3) and five on the west side (38 to 42), connected by loop tracks south of the concourse. The tracks begin north of 43rd Street and remain parallel as far north as 48th Street  Through the two separate throats, the number of tracks reduces to six on the upper level  by 52nd Street, and to thirteen on the lower level by 51st Street. The tracks on the lower level rise up to the same level as those on the upper level by 55th Street, with more crossovers reducing the total number of tracks to four in the tunnel under Park Avenue by this point. This entire line is third-rail electrified, operated by TCS (CTC), and heads north-northeast. North of the crossovers, there are platforms on each side, recessed into the tunnel walls, labeled "Emergency Exit, 57th Street."

Grand Central Terminal's stub-end is at MP 0.0, with the upper level and lower level throats at MP 0.4, all with speed limit 10 mph. There are more crossovers at CP1 (MP 0.7), where the speed limit rises to 60 mph,  a speed restriction to 45 mph on Tracks 4 & 3 (the outer tracks) between MP 2.1 and MP 2.2 (86th Street), and between MP 2.6 and MP 2.7 (95th Street), a speed reduction to 45 mph on tracks 2,1 and 3 at MP 3.0, and crossovers at CP 3 (MP 3.2), the latter after the tracks have emerged from the tunnel and risen, through a stone cutting, onto a viaduct above Park Avenue. There are 20 bridges over streets, a speed reduction to 40 mph on all tracks at MP 4.0 (with a restriction to 30 mph on Track 3 between MP 4.0 and MP 4.3), a station with two island platforms serving the four tracks, above street level, at 125th Street (MP 4.2), with a bridge over a street within the station, four more bridges over streets, a turn northeast, a speed reduction to 35 mph at MP 4.5, two parallel through-truss movable bridges over the Harlem River at CP 4 (MP 4.6), including a bridge over a rail line running along the north side of the river and a bridge over a multi-lane divided highway, a speed rise to 40 mph at MP 5.0, two road bridges overhead with the tracks in a cutting with stone retaining walls, crossovers at CP 5 (MP 5.2), where the four-track Harlem Line continues northeast and the three-track Hudson line swings north-northwest, along (but not yet on) the north side of the Harlem River, a Maintenance of Way yard with at least four tracks on the west side, and the west apex of the wye with the Harlem Line at Mott Haven (MP 5.4).