The QuickTrak machine is located in the EWR train station, after you get off the AirTrain. In an ideal world, it will recognize you through any credit card. If not, you can use the true train number (which you can look up on amtrak.com) or the Amtrak codeshare flight number (which appears on your confirmation). Or you can call Amtrak and get your Amtrak PNR and use that.

If you'll be in an Amtrak train station anywhere in the country during the day leading up to your flight, you can also get it there.