Hi all, I'm writing this form Wengen. I promised I'd come back and update you how it all went.
So I purchased a Swiss Card before heading to Venice. We planned to travel from Venice (via Milan) into Sion, Switzerland.
TrenItalia would not validate a Swiss pass. What they did was only charge me the fair to the last Italian station (Domodossola).
The train I boarded in Milan would take me straight into Sion, Switzerland. So there was no way of validating my Swiss Card at a SBB train stop.
When the conductor came on board I was a little nervous to be honest. I told him that the Italians didn't want to validate the card and I had no other choice but to wait for you (the conductor) to come on board.
He told me it could have been a $100 CHF fine per passenger. But he watched me fill up my Swiss Card and validated it on the spot.
I didn't argue!
Seems like it's not preferred, but also seems like he has seen enough of these scenarios not to surprise him.
So, there you have it. You can do it on the train. Just pray you get a nice conductor :P
Lastly - a funny thing happened in Domodossola (Italian border station). When we stopped, I thought maybe I'd be proactive and look for an SBB conductor. So I took my white envelope containing my Swiss cards and moved to the back of the cabin. For some strange reason I decided also to take a leak first in the cabin toilet. At this very moment, the Italian border guard was coming up the train - he saw me carry a white envelope and head straight into the toilet. Talk about awkward.
I didn't manage to find an SBB conductor. What happened next was the Italian border guard walked over to me and asked me a whole load of question, with a suspicious glare. He had obviously caught me walking into a toilet with a white envelope just as they were about to do inspections - so I don't blame him. He asked if I had anything to declare and what was in my white envelope, where I was going, etc.
LOL - it was kind of nerve wracking to say the least. Lesson learnt - don't move around at a border station, and definitely not when the border guards are coming onboard the train!!