Just use mobile data? That's what the trains are using anyway.
I would think the train can have a much more powerful antenna to talk to mobile towers and then relay that to the passangers. So that WIFI is better than mobile data.
This report[0] says it's 62-78% nationwide. Of the neighbouring alps countries Austria is 78-88% and Switzerland is 93-96%.
[0]: https://www.umlaut.com/uploads/documents/Reports-Certificate...