Checking in for UA code share segment on Amtak?
 

Can someone tell me how I actually get an Amtrak ticket/BP in EWR when making a code share connection? I know UA says to use the QuickTrak machine but I have no idea what I'm supposed to enter into the machine at that time. TIA.

A credit card and the codeshare UA number.

Or the train number, if you look it up on amtrak.com.

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.