There is nothing you need to do, I would suggest carry all documents that you would for stamping. Keep all your I94 with you, as you approach the CBP tell the officer you want to use AVR. Now if you take the train, may get tricky. Most likely you will go into secondary, they are under no obligation to clear you before train departs. One my friend took the train few years back, he boarded the train at Pacific station. Then at the border the CBP officer come onboard and do the immigration. Since most likely they may send you to secondary in other words grab your stuff and deboard. If that happens you will most likely miss the train. Now there was talks going on of doing the preclearance at the Pacific station itself. If that's how it works now then it's very good for you.