Hello BilboB
You can also buy the ticket at www.sbb.ch/en or http://trenitalia.com/en.html.
For the Switzerland to Milan travel, the Swiss Travel Pass is valid as far as Chiasso (via Gotthard route) or Domodossola (via Simplon route).
However, holding the Swiss Travel Pass, you can book a trough ticket from Zurich HB to Milan. While buying this ticket, at a certain step you can declare the “GA Generalabonnement” as a reduction type. By doing so, the ticketing system will know that you hold the Swiss Travel Pass, consequently the Swiss stretch will be free of charge and only the Italian part will be payable. But, you will get seat reservations for the entire journey - very convenient.
Note:
- the GA Generalabonnement is the resident’s equivalent of the Swiss Travel Pass;
- the instructions above apply to GA and Swiss Travel Pass holders only, not to any other day pass or other tourist offer.
Kind regards,
Roland