Hello SarahN0704
Is it planned to change in Lugano on the RE train to Milano Centrale? So, you may have to split the booking. First buy the part Lugano to Milano Centrale with the RE Transfrontaliera and then the train ticket from Zurich to Lugano. Thus, the booking should work out. An EC on the other hand can be booked directly from Zurich HB to Milano Centrale. Thereby only a message can come if the train is already fully booked.
For more information, please call the Contact Center at 0848 44 66 88 (CHF 0.08/min.).
Thank you for your understanding and have a nice day.
Kind regards,
FatimaH