I tested FreeSurf for the first time in my life today, just for you 
I have two SIM cards in my smartphone:
| SIM | Provider |
|---|---|
| 1 | Yallo, unlimited Internet |
| 2 | Swisscom Prepaid, CHF 2 / Day |
I proceeded as follows:
- Enable SIM 1 for data and SIM 2 for sms/phone only
- Download FreeSurf-App
- Register with number from SIM 2
- Tab on FreeSurf start, wait for confirmation sms
- Switch data from SIM 1 to SIM 2
Result: According to my IP address, I’m surfing with Swisscom, and the usual “you have just paid CHF 2 for 1 day of internet” sms hasn’t arrived yet. But since I don’t know how much money I had on my prepaid sim card before, I can’t say whether anything has been charged so far. The Swisscom online customer center says consumption is 0, but I don’t know if that’s in real time.
But I can confirm: Activation is only possible with an internet connection. That might be a job for Firsty or for a Digital Republic for CHF 4 / month (unlimited, throttled 0.4 Mbit/s).
The FreeSurf app detects the train using Bluetooth beacons. It would be interesting to know whether such a Bluetooth beacon can be cloned. Then you would have free internet access throughout Switzerland, even without a train 