Comment by seanmcdirmid

Wiki says it was more because of the private right away used, not necessarily its steepness:

> While many streetcar lines were converted to bus lines after World War II, the J Church avoided this due to the private right-of-way it uses to climb the steepest grades on Church Street, between 18th Street and 22nd Street.[9]

https://en.wikipedia.org/wiki/J_Church