The S4 is a commuter rail route forming part of the Milan suburban railway service, which converges on the city of Milan, Italy.

The Milan–Asso railway is a regional railway line with standard track gauge which links Milan to Canzo crossing for Erba and other towns in Brianza. The most northern terminal is the station of Canzo-Asso, which is located in Canzo's territory but is also known as Asso in the short form. That, because there is another station on the line called Canzo station and Canzo-Asso is next to Asso's boundary and serves this commune too.