Brown rats are the type that are very good swimmers and they therefore can traverse and live in sewer systems, and often do in urban locations around the world.
Other than because of their excellent swimming ability, the reason these rats like to live in the sewers is because there is a steady supply of food and water there, making it quite comfortable for them.
Rats are true omnivores and they can eat almost anything, and there is plenty of refuse in a sewer for them to subsist on.
Sewers also provide protection against most of the rats natural predators, as there are not many other animals that can thrive in a wet, underground environment like this, with snakes being the notable exception.