www.seat61.com for the best information on train travel I know.
www.ricksteves.com for his Travel Tips: Thefts and Scams.
Some guide books. I'd suggest Lonely Planet or Rough Guide for their age. Maybe downloads to their devices. The guides have maps, directions, history, and all sorts of survival information.
Tell them to get their currency exchanged at ATMs, with cards having minimum costs. There is a whole section on this elsewhere on FlyerTalk.
Get them some secure container for their most important items - passport, credit cards, major cash, even critical medications. Either a neck pouch or a money belt. Attached to their body, and more safe from pick-pocketing. Explain that what goes in it should be enough to get them home if
everything else is lost or stolen. And it doesn't come off their body until they are safe in a hotel room. Or if they are planning on staying in hostels, there are lockers available.
I think the luggage allowance on the European Low Cost Carriers is 10 kg or about 22 pounds. Even if they can carry more on the flights from the US, if they keep it to this level they then won't have issues with European flights. But do check that allowance number first.
Have them learn what they need to charge those electronics and keep those phones working in another country. Different plugs, different current. I use a minimalist approach with an iPad mini with WiFi and email. Sometimes the hunt for WiFi is a challenge, but I always eventually find something. I used to rely on just finding an internet cafe. With phones one gets into the Sim Cards and the different phone numbers for placing calls. Whatever they choose to do, they need to figure out how it will work in the countries they think they will visit. Tip - the backs of some TVs have USB, and one can charge devices that way - slowly.
If they've studied certain languages in high school, the countries speaking those could be good choices.
Romelle