Dear Dr. Boli: Why did I just see a Red Line car on the Blue Line in Overbrook? I mean, you expect to see the Silver Line on the Blue Line, and sometimes the Red, Silver, and Blue Lines on the Brown Line, but why the Red Line on the Blue Line? Is this a sign of the End Times? —Sincerely, The Rev. Albatross Tarn, Countdown Assembly of God Church Congregation, Overbrook.

Dear Sir: Your question gives us an opportunity to explore the deepest workings of the bureaucratic mind as it interacts with the 163 years of encrusted streetcar tradition in Pittsburgh. Bring a compass, climbing tackle, blasting caps, and a ukulele, and we shall begin our descent.

The general rule is that the Red Line begins at Overbrook Junction and winds under Mount Lebanon, through the back yards of Dormont, and down the main street of Beechview, finally joining the Blue and Silver Lines at South Hills Junction to go through the Transit Tunnel and then to the subway downtown. However, a fair number of the inbound Red Line trips actually originate farther out at South Hills Village (because the car barn is there) and follow the Blue Line until the two lines diverge at Overbrook Junction; and likewise a number of the outbound trips go past Overbrook Junction and continue to South Hills Village.

A few days ago, some shifting was detected in the long viaduct that carries the Red Line and the South Busway (the Yellow Line on the maps) over the Saw Mill Run valley. For certain historical reasons, Pittsburghers are in a mood to worry about bridges at the moment. Till the bridge is repaired, the inner half of the Red Line is closed from Potomac on in to South Hills Junction. Rail shuttles run from Overbrook Junction to Potomac, and bus shuttles through Beechview to downtown.

So far that probably makes perfect sense to you. However, there are still the Red Line trips that originate or end at South Hills Village. These are now diverted over the Blue Line. For the entire route, they follow the Blue Line stop for stop. There is nothing but the words “Red Line” on the front of the trolley to distinguish them from Blue Line trips. They are, however, still Red Line trips by nature. It is true that, from the passenger’s point of view, they have nothing at all to do with the Red Line. The naive passenger thinks that what makes a Red Line car is the fact that, sooner or later, it travels on the Red Line. The trained bureaucrat, however, sees through to the true εἶδος of each trip, and understands that, even if the Red Line is temporarily closed, even if the Red Line were to be wiped off the map by map-wiping weevils, cars that set out from South Hills Village at the times set aside for Red Line trips are still Red Line cars. This is why you will be seeing Red Line cars running through Overbrook until that viaduct is put back together properly.