Originally Posted by
JMN57
https://www.trenitalia.com/en.html
Punch in Malpensa for departure and Naples for destination on your day and you can see the options. Link is for English version. Useful to download the app.
The trick is to take the train to Milano Centrale station from the airport, not the faster train to Milano Cadorna station. The high-speed trains for Naples depart every hour from Milano Centrale.