Hello,
I'm a rail fan from Turin. The Porta Susa area is undergoing a major redevelopment. The old surface rail station is now closed, and the new underground rail station (FS) is in operation. Trains now stop at the underground station (including the TGV from Paris).
The link between the underground train station and the metro station (Porta Susa Metro Station) hasn't been completed yet, so it is not possible to board on a metro train at Porta Susa. The nearest metro station is Piazza XVIII Dicembre, which is in fact the metro station for the old Porta Susa rail station.
It sounds complicated, sorry, but in fact it isn't! Once you have got off at the Porta Susa rail station, you just follow the metro signs and you will be directed towards the XVIII Dicembre metro station. It is a 3 to 5 minutes walk, depending on how heavy your luggage is! The walk is entirely within the rail station area.
Underground trains are pretty frequent (from 3 to 5 minutes), and the journey from XVII Dicembre to Porta Nuova station takes approximately 5 minutes. The underground is operated without drivers, as in the case of Lyon, Lille and some Parisian lines.
If you need any additional information about travelling in Italy by train, please let me know. I am happy to help.
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Sorry, I forgot to say that a 15 minutes connection at Milan Central should be avoided at any cost!!