Yes, it stands for Pennsylvania System.
"Pennsylvania System" was the service name used to designate the combination of Lines East and West. You will find it on certain types of advertising matter, and in the names of several joint bodies that had members from both halves of the railroad.
The "Lines West" organization was abolished in 1920, although it took about two more years before all of the properties were actually leased to the PRR. At that time "Pennsylvania Railroad" replaced "Pennsylvania System" as the name for the whole railroad. This is what you see on post-1920 timetables, advertisements, etc.
