According to Stefan Lorant's comprehensive history on "Pittsburgh, the story of an American City" William Flinn was one of two men who ruled the city during the 1880s. His method of operation was to assure that specifications for public works would be worded so only his company, Booth & Flinn, would qualify for the contracts: "the stones for public buildings were specified so only his company could supply them; asphalt for the streets had to be of a quality that only his firm possessed." Flinn, along with Christopher Magee, kept control of the city by giving their supporters their share of the "Take" from the vice and corruption. Dissenters produced evidence that Booth & Flinn of construction practices that did not meet the contract specifications and resulted in inferior products that, of course, needed repair and replacement long before that should have. These complaints were made to no avail. The machine disintegrated after the 1901 death of Magee, but no more mention was made of Flinn. One can probably safely assume that he acquired the charter for the Street Car lines to the South Hills in the same manner.
I just wonder if he also manipulated the city into paying for the construction of the tunnel. It is surprising that it lasted as long as it has considering the reputation of Booth & Flinn.