In my defense, the situation around the voucher system seems to have confused most everybody. This sentence, however, might not make any sense because it looks like craz deleted their reply. That's unfortunate because it saved me a bunch of time today.
What I think I learned today is there appears to be two totally separate ways to end up on the high speed rail. In either case, if you land at Natbag Terminal 3, you exit customs, go out exit
#3 (IIRC) and turn right.
1) If you have a Rav Kav card, you use to enter the station, go downstairs, and take the train to Jerusalem Yitzhak Navon station. The trip is free.
2) If all you have is a credit card or shekels, you go to the machine (outside the station), pay 17 shekels, get a ticket, use the ticket to enter the station, go downstairs and take the train to Jerusalem Yitzhak Navon station.
I don't quite understand where you can get the Rav Kav card from, but one of the locations is Yitzhak Navon station where it costs 5 shekels. You can then ride back to the airport for free.
I'm sure I've got some of this wrong, so please correct me.