Train travel to Cinque terre

That should be enough time. Be ready to de-train as soon as it stops, and walk toward the front of the train to find the track for your CT train on the big board overhead. Milan is an "end station" so all the trains come in to the end and you can cross between platforms at the head of the trains, no tunnel crossing necessary. The tracks are all parallel and are numbered 1 to 24(?) from left to right as you stand facing the trains.

Ditto Lola. There are 24 parallel tracks that deadhead into a common platform. Trains from Arth-Goldau used to come in on platforms 4-6. Trains to the CT used to depart on platforms 18-23. Should be a 5-7 minute walk between trains. As Lola suggests, check the large digital signboard as soon as you can. This information could have changed.

Barbara, As the others have mentioned, 20-minutes should be more than enough time. Don't stop for coffee though! When transiting Milano Centrale, I've found that they don't always display the track numbers of departing trains on the electronic board until shortly before departure. This may or may not happen in your case. I find that it helps to make a note of the train number that you'll be taking (ie: EC-873), as that makes it easier to confirm the information on the electronic board. Happy travels!

Barbara, As this is your first trip to Europe, hopefully you've now read Europe Through The Back Door from cover-to-cover? If you live reasonably close to Edmonds, stop by there one day and peruse all the Guidebooks in the free Library there (during business hours). While there you can also have a chat with Rick's incredible staff, and sort out any other concerns you might have. Cheers!

THANK YOU SO MUCH!! This is my first trip to Europe--lots to learn!