I'm writing a novel that plays in 1945 Vienna. There are characters who want to get from Währing (A in the image below) to the city center (B) undetected.
Is this realistic/believable for them to travel this route underground (in the sewage system; occasionally getting to the surface)?
My answer: Yes.
Justification #1: The modern sewage system covers most of Vienna (see below).
Source: Viennese government
Justification #2: Both the starting point and the destination of the journey are located in old parts of Vienna. According to German Wikipedia the first sewage system that covered the entire city was completed in 1739. If I assume that the building in Währing and most of the buildings in the city center were built somewhere during the 19th century, it is reasonable to assume that they were connected to the sewage system in 1945.
Justification #3: Both Währing and the city center are wealthy districts. It is reasonable to assume that people in these areas got the canalisation first, before inhabitants of poorer districts.