From the crossover at CP-Burn (MP 0.8), where a line to Bethlehem via East Penn Junction departs on the south side, and the Reading Line turns northeast, bridging over an RJ Corman line and the Lehigh River, and the junction with the Lehigh Line at CP-Allen (MP 0.0/88.2), where the Lehigh Secondary Track trails in on the north side, and there is a crossover, the Lehigh Line turns east, single track, Rule 261, maximum speed 20 mph, past the start of Allentown Yard on the north side at CP Canal (MP 86.9), turning northeast, east, southeast, and east again, passing Steel (MP 85.5), to the end of Allentown Yard, crossovers, and the Cement Secondary Track and Freemansburg Industrial Track starting on the north side, at CP-JU (MP 84.5), where the maximum speed drops to 10 mph, and turning southeast, bridging over the Lehigh River, to the junction with the additional segment of the Reading Line at CP-Bethlehem (MP 84.3), where the Lehigh Line trails in on the north side.
From CP-Burn (MP 92.8), the line off the Reading Line heads east, single track Rule 261, maximum speed 25 mph, past East Penn Junction (MP 92.6), where an RJ Corman line from Catasauqua trails in on the north side, the East Penn Yard starts, and the maximum speed rises to 30 mph, east-southeast, and then east-northeast, past Riverside, and a detector at "Bethlehem" (MP 91.3), to the junction with the Lehigh Line at CP-Bethlehem (MP 88.6), where Double Track, current-of-traffic (Rule 251) starts, maximum speed 40 mph the Lehigh Line trails in on the north side (10 mph on the connector), and there is a crossover, heading just north of due east, past the start of an extra track on the north side at MP 88.6, a crossover, and the access to Florence Yard (MP 87.0), where the erstwhile Bethlehem Steel plant used to be (and has since become a Norfolk Southern intermodal yard for the northeast area).
The extra track on the north side ends at Freemansburg (MP 85.8), where the erstwhile Reading line from Quakertown once trailed in from the south, the yard ends, and the line turns east-northeast and then northeast, curvily east-northeast past detectors at "South Richards" (MP 82.8), a crossover at CP-Richards (MP 81.1), where Rule 261 replaces the current-of-traffic (Rule 251) on both tracks, east, past MP 79.5, where the maximum speed drops to 30 mph, Glendon (MP 78.6), a line from Keplers Mill bridging overhead, Abbott (MP 77.9), the start of single track, Rule 261, at CP-Easton (MP 77.0), where the Portland Secondary Track heads away on the north side, bridging over the river separately, the bridge over the Delaware River (and thus the Pennsylvania-New Jersey border) at MP 76.6, and over the routing of the erstwhile Jersey Central line at this point, and the junction with the Washington Secondary Track (former Lackawanna), on the north side, at CP-Phillipsburg (MP 76.3), where the Lehigh Line turns south-southeast.