The wireless device works when it can see the computer running calibre. Usually this is at home, unless you do something very special.
If you want train access then the SD card may be the best bet. That said, you might still want to connect from time to time as a wireless device in order to update metadata of books you already have on the device.
If you are using the SD card "Cloud Connection" you might also find that the demo version of CC does everything you want.
