The construction of the Mount Washington Transit Tunnel was instrumental in the development of the South Hills, as it shortened travel times to Downtown Pittsburgh significantly.

[1] The Mount Washington Transit Tunnel is used by light rail trains and local, regional, and express bus services.

As development progressed, demand grew for better transport options over Mount Washington, a significant geographical obstacle between the coal-rich South Hills and the city.

The Pittsburgh and Castle Shannon Railroad began operating through service across Mount Washington in 1874, using two pieces of former coal transport infrastructure.

Trains through the tunnel connected passengers to the Pittsburgh and Castle Shannon Plane on Mount Washington's northern face.

2 on the respective southern and northern slopes of Mount Washington, removing the need for the narrow, dark, and smoky tunnel.

Cable cars and electric trolleys debuted in the city the late 1880s, causing a revolution in land use and transport policy, but by the end of the 19th century, the inclines on Mount Washington were the best technology for their geography.

The P&CS chose not to electrify its lines, as it was a vertically integrated company which produced high-quality coal from mines it owned.

[5]: 9 At the turn of the 20th century, two related but competing firms sought to build a tunnel for trolleys through Mount Washington.

In 1903, Pittsburgh Railways intervened and took control of the project, investing millions of dollars into the Mount Washington Tunnel Co. and settling the dispute with the Pittsburg Tunnel Co.[8] Construction proceeded at a rapid pace, with workers working six days a week to bore through the solid rock of the Casselman Formation.

[14] Ridership briefly rebounded during World War II, but by the 1950s the Pittsburgh Railways system was growing smaller.

Local services through the tunnel continued, as Pittsburgh Railways converted more of its city trolley lines to buses in the 1950s.

The trolley built up speed through the tunnel and left the rails at the north portal, riding on the pavement until it came to a stop on the southern approach of the Smithfield Street Bridge.

[10]: 5 From the northern portal at Station Square, the tunnel rises through the entire Casselman Formation, emerging at South Hills Junction.

[5][10]: 5  In both instances, trolleys entering from the south tunnel ran downhill, lost control, jumped the tracks at the north portal, and crashed.

A fatal incident occurred on Christmas Eve 1917, when a trolley on the Knoxville line crashed at the north portal of the Mount Washington tunnel.

14 people died immediately, and the remaining wounded were taken to the nearby Pittsburgh & Lake Erie Railroad Station Terminal Annex, which served as a makeshift hospital.

The operator, realizing the car could neither stop nor take the sharp curve from the transitway to the Panhandle Bridge ramp, ordered all the passengers to move to the back, and radioed the PAT central dispatcher to clear Station Square.