South Hills Junction became a reality in 1904 with the completion of the Mt. Washington Streetcar Tunnel. According to an article in the Pittsburgh Press on April 15, 1958, the Wabash Railroad began digging a tunnel to connect to its downtown station in the early years of the 20th century. While skeptical, the city's trolley financiers watched the progress carefully. As soon as it became obvious that the Wabash Tunnel could and would be successful, plans were made for a trolley tunnel. One of the pre Pittsburgh Railways companies was owned by a real estate developer named William Flinn and his partner Frank Booth. They were determined to beat the Wabash through the hill and started digging, or blasting, their tunnel on April 6, 1903. Meanwhile bridges and trestles were built to complete the routes. They didn't beat the Wabash, but the digging was finished on Oct. 7, 1904. Then 12 million bricks were laid, track installed and electrification completed. The first celebrity filled car clanged through on Dec. 4, 1904. The Mt. Washington Trolley Tunnel climbs on a 6% grade for a distance of 3500' from Carson Street at the Smithfield Street Bridge to the back of Mt. Washington near Warrington Ave.
At the south end of the tunnel South Hills Junction grew, starting as a straight two way junction with one branch going to Beltzoover and Knoxville (routes 44-48) while the other branch headed for Mt. Lebanon, Brookline, and Dormont (Routes 38-39-42). (I don't know when the 40 Mt. Washington line was built). Electrification and the placing of extra rails outside the narrow gauge [on the Overbrook line] began in October 1908 and the first electric cars were run inbound through the valley route on August 17, 1909 after a long battle with Mayor Magee . Outbound service was established on November first of the same year. This route joined the existing lines at South Hills Junction. It is believed that the P&CSRR stopped hauling coal in 1912.
In 1907 Swartz and Dawson submitted an estimated cost of $9,235.64 for the construction of a 35' x 60' two story brick building, dispatchers office and waiting room. Changes to this building were numerous but not documented. Go to the timeline page to see an attempt to show the modifications.
In 1906, a single track was laid behind the Administration building connecting the 44-48 lines to the 38-39-42 lines. This was done so 44-48 cars could return to the car house without going through the tunnel. A couple of spurs were eventually placed on this "back track", one with a double cutback, possible to a rock crusher on the hill. In 1927 one of these spurs was connected to the Overbrook line , and I believe that between 1935 and 1937 the other was used to make connection to both the inbound and outbound 44-48 lines.
In 1930 a loop was put around the front of the building allowing inbound the interurban cars to get to the shop area. This was necessitated by the closing of the Castle Shannon car barn. The loop also permitted a car inbound on any line to go to any other line outbound.
A single story watchmans shanty was erected at the Junction to control routing and probably signals. In 1909-10 derails were installed and the watchmans shanty raised by 12'. In 1917 plans were drawn for a two story watchmans tower at this site. To me this drawing and later pictures look like the original shanty, rotated 90 degrees, and elevated 12 feet. Maybe the plan was documenting what was done. The watchmans tower controlled the junction's switches as well as the derails and signals. I found an operations directive, detailing the signaling and procedures. . The tower was removed when the Junction was interlocked in 1939. The Port Authority, in their infinite bureaucratic wisdom, removed the derail when the tunnel was paved for buses (government owns it now, therefore there will never again be an accident).
There were other changes to the junction area, some documented and some gleemed only from pictures and maps and memory. I'v attempted to put together a timeline of these changes.
For a first hand view from a real railfan who lived above the Junction see Bob Schmidt's account.