I’ve just come back from Milano yesterday!
I didn’t do much rail based, but did travel by train to Lugano in Switzerland which was incredible. About 1hr15 mins away, but we got held up at the border in both directions (on the way back for 45mins). You can get the more expensive EC train (which goes to Zurich) or the regional Trenord service - this can get exceptionally busy between Como and Milano. (You can tie in a trip to Como with Lugano, as the train service stops at both).
At Lugano there is an interesting inter-urban tram line S60 which goes up to the Italian border. There are also two funiculars to the mountain tops south and north of Lugano. I used the funicular to Monte Bre which was well worth the visit. (Sorry, I got carried away, this might not be relevant to your question, but useful to know for any future trips!)
Back to Milano, I only really used the trams and trolleybuses. The elderly 1930s Peter Witt trams were still very common which surprised me, on intensive all day service right up to past midnight, and were not just aimed at tourists, as they work some very ordinary routes. Though of course they were popular with tourists, but not to the overbearing extent they are in Lisbon. The Peter Witt trams were the full allocation of routes 1, 5, 10, 33 and partially on route 19, though there may be more routes they operate. It was fantastic to see these in normal everyday operation - while it’s clear they’ve had some modernisation, they don’t feel hugely different to when they were original (presumably!), and the noise they make is just wonderful. The drivers certainly drive them fast, and be careful with the doors as they tend to open them before the tram has even stopped!
If you are a tram fan, it’s not just the Peter Witt trams I found of interest, but also the 1950s/60s ATM 4700s which I saw a lot on route 2 (I actually slightly prefer these to the Witt trams). There’s also the 1970s 4900s, mostly on route 3 it seemed.
I didn’t find the Metro particularly interesting myself, but it was a much quicker way to get around than the trams (though I preferred the slower tram journeys).
There are also trolleybuses, on orbital routes 90, 91 as well as 92 and 93 but these don’t go near the main city centre.
In terms of rail, there is an underground S-Bahn style line that goes through the centre, but I didn’t see this myself. Milano Centrale is perhaps the most impressive station I have been to anywhere. The scale of it is just incredible. It’s worth having about an hour there just walking around looking at the architecture. Sadly though you are not allowed at platforms without a ticket.
It felt a lot safer than many other big European rail terminals too - lots of security, police and armed soldiers.
A smaller terminal of interest may be Porta Genova by the Navigli canal district (this being an interesting area to walk around anyway) which has a traditional feel about it, and you can see where the tracks used to continue further
