[1] The core of the system is the Passante, an underground railway running through the city approximately from the north-west to the south-east.

Since 1 October 2019, in provinces that include only municipalities where new STIBM tariff system[14] (introduced on 15 July 2019) pricing is active, IVOP titles have been abolished.

[15] Several train classes are operated on the network: Treno ad alta frequentazione (TAF) class trains, that were introduced since the end of the 1990s and are mostly unused as of 2024; Treno Servizio Regionale (TSR) class trains introduced since 2007 to replace older rolling stock; Caravaggio introduced since 2019 and mainly used on the S8 and S11 lines; ATR115 and ATR125 (based on the Stadler FLIRT platform) only used on the S7 line; and the E.464 locomotive with "Carrozza vicinale a piano ribassato" used on the S9 and S8.

Line S8 and S11 began operation at the end of the same year, while the service was extended to reach Lodi and Treviglio.

On 1 May 2011 a new company, Trenord, was created from the joining of the two main regional train operators in Lombardy, Trenitalia and LeNord.

[18] In the following the EXPO event (that brought international attention to the city) a new S12 was created to help with crowding during rush hours (8 runs a day) and 2 stations were suppressed (even though every diagram says they're temporarily closed).

With time everything got back to normal except S12 which was reactivated only at the end of August 2023 and same goes for the half an hour frequency for the S9 line.

[21] At least five new lines are expected to enter service in the next few years, with some of them needing to wait for the completion of new infrastructural projects in order to be activated, such as the quad-tracking of the currently double-tracked Rho-Parabiago section on the Milan-Gallarate railway:[21] Several new stations are also planned to be built to better serve the Milan urban area, as detailed by the Plan for Sustainable Urban transportation (or PUMS in Italian)[1] for the municipality of Milan.