Doug,
So when you arrive at Milano Centrale, you have plenty of time to check the electronic board to find out which track your train will be leaving from. Sometimes the train may be on the track 15-20 minutes before departure, so if your train is there, you can board it & just wait.
Make sure you board it at least 5 minutes before because these trains tend to leave right on time.
We missed a train going to Varenna & we had to wait an hour for the next one...but when our train was just leaving, there was a young couple running to the train but they were too late & they had to wait for 2 hours for the next one...