The purpose of the Chartiers Branch was to connect Washington, Pennsylvania to the PRR's main line in Mansfield (later renamed to Carnegie), Pennsylvania. It is a 23-mile branch that runs in south/southwesterly direction in the area southwest of Pittsburgh, Pennsylvania. In the map, it is the right-most near-vertical red line.
The branch was named after the Chartiers creek, which it follows and crosses a number of times. The Chartiers creek flows from its origins all the way north to the Ohio River just west of Pittsburgh. The name "Chartiers" came from a half-French, half-Native American by the name of Peter Chartier, who settled near the watershed of the Chartiers Creek and built himself a trading post.
The map a portion of the one that came in the Pennsylvania Railroad Historical & Technical Society's quarterly magazines, The Keystone, Vol.48 No.4. The map is from November 1941 and is from Chuck Blardone's collection. However, the branch line hasn't changed much from when it was first constructed, and it still exists and is still being used today. The PRR's Chartiers branch is labeled as the "P.C.C. & St.L. R.R.", which was the PRR-owned operating company that ran that line.
In this map, also from November 1941, I have labeled some of the key points along the Chartiers Branch. Follow the heavy black line from Carnegie down through Bridgeville, Morganza, Canonsburg, Houston, and down to Washington. That is the Chartiers Branch itself.
At Carnegie, PA, the branch starts at 770 feet above sea level. At Washington, PA the elevation is 1,060 feet. Most grades were less than 1%. The track follows in the valley of the Chartiers Creek. The reason why I became interested in modeling this branch was the fact that it crossed the Chartiers creek 19 times in the 23 miles, and it included one 500-foot long tunnel. Outside of the towns the scenery is mostly rolling hills with lots of wooded areas; just my cup of tea.
(In the diagram there are two labels "start" and "end" "of modeled area". This was for my first S-scale layout, so just ignore that.)