Train from Paris to Milan

As of 2021 there is no overnight on that route.

There is however several direct day trains.

I did back in 2019 and would never do it again. It was a rough ride with a 6am arrival. Tough night of sleep. If I am ever in that position again, I will fly.

The day trains are a choice between TGV and the Italian Freccariosa trains, on their new route to Paris competing with SNCF. If you book ahead all reviews say that the stylish Italian trains are the way to go, especially in Business class. And that the day trains beat flying hands down in terms of service and scenery. Sure it's a 7 hour journey, on the outer edge of the time gains from flying, but sometimes time is not everything.