Carnegie, PA was incorporated in 1894 by the merging of two boroughs, Chartiers and Mansfield. These two towns were separated by the Chartiers creek. The Pennsylvania Railroad had Scully yard in that area, which is from where the Chartiers branch breaks off.
If you follow this map (showing where the Scully Yard used to be in the inside corner of the Chartiers Creek bend) down south, you will find the view shown here. That is where the Chartiers branch actually starts (as far as it shows today). The Chartiers branch breaks off to the right (east) and then starts its journey following the Chartiers creek. The topography map, copied below, shows the same area near the bottom (center) of the map, with Scully Yard being visible at the top (center) of the map.
From Bob Ciminel's web site: "The crossing at Campbells Run Road was a busy place as trains went by on the Panhandle main and onto and off of the Chartiers Branch. I remember the abandonment of the Chartiers Branch connecting track at the Carnegie passenger station happening in the early 1950s. The 1955 Panhandle Division Employee Timetable shows the trainphone was in service on the Chartiers Branch between Wagner Interlocking (Carnegie) and MO Manual Block Station (west of Hills at Milepost 9.6). That would indicate that trainphone-equipped cabin cars could and would operate on the Chartiers Branch."