Milano has 4 subway lines:
Red Line 1: Sesto F.S. - Rho Fiera Milano / Bisceglie
Green Line 2: Gessate / Cologno Nord - Assago Milanofiori Forum / P.za Abbiategrasso Chiesa Rossa
Yellow Line 3: Comasina - San Donato
Lilac Line 5: Bignami Parco Nord - San Siro Stadio
Line 4 is under costruction.
To the subway lines is added the railway Passante, which integrates the transport in the city making it even more widespread.
Download the map of the public transport system