Line S9, a cross-city route, initially heads in an southeasterly direction from Saronno towards Cesano Maderno, Seregno, Monza and Milano Greco Pirelli, then runs via the Milan belt line and southern belt line around the eastern and southern sides of the Milan city centre, before finally heading southwest, to Albairate-Vermezzo.

[2] The S9 commenced operation on 24 December 2004, and initially linked Seregno with Milano San Cristoforo.

[3] Thanks to the activation of the S9, local passenger trains returned to the southern belt line after an absence of 10 years.

Initially, the S9 was managed by TiLo, a company formed ad hoc by Trenitalia and the Swiss Federal Railways.

Upon the timetable change on 13 December 2009, train services on the line became half-hourly throughout the day.