If you’ve got the cash, get a mobile phone that has good rural and city coverage. I’m unsure about the US but here in Canada both CP and CN have 800 numbers that shippers can use to track cars (though not leased cars, these have an “X” in the serial number). Of course if you happen upon a seasoned hobo they might share some knowledge with you.
The people you’ll want to watch out for are usually the bulls. They’re hired to make sure nobody is painting, stealing or riding. Most brakemen and switchers are pretty laid back though of course you can never rely on that.
Bring something to cover your face and eyes. Depending on the length and layout of the track you’re on, some trains can get moving very fast and they kick up an immense amount of garbage. Some grain hoppers have holes in either end of the car that you can fit inside of and that can shield you from some of it. To be honest I don’t know what they’re for but it doesn’t seem like anything goes through them so you should be safe. Food and water should be givens.
If you’re seriously going to try it be bloody careful in those yards. Whenever possible, avoid crossing lines between cars. You never know if they’re going to hump them (not sexual humping but slamming them into each other when they sort them). The best bet is to find the end of a line of cars and go around there and give it room. You’d be surprised how quiet those beasts can move with no engine nearby. Also be aware when you’re in between two lines with cars. If they’re moving in different directions it can trigger serious dizziness in some people. The last thing you want to do is fall over with two massive trains on either side of you.