system
Given the following post, respond as if you're commenting on Reddit:
human
[Fallout 3] What’s the state of international travel in the Fallout universe?

In Fallout 3 there are at least two characters who have travelled from Europe to The Captal Wasteland, Moriarty & Dukov. Neither seems the type to make a voyage of discovery, so they must have known to some certainty what to expect when they arrived. 

How would they have travelled there and what kind of international infrastructure exists?
gpt
I don't believe we get a hard answer, but the Far Harbor DLC from Fallout 4 does show that people are operating powered fishing vessels on the Atlantic with some success. There are dangers; storms are still a huge risk factor, as well as hostile FEV-altered aquatic life. While it doesn't look like international trade had been re-established, it does tell us that small vessels can traverse the ocean, if someone is willing to roll the dice. Of course, not many do, since America is still kind of shot to hell, so you only see a random smattering of folk who felt a mighty need to GTFO of Europe instead of any significant gatherings of Continental or UK folk.

As far as knowing what to expect, they'd have some clues. American and European fishermen could be crossing paths up by Newfoundland or Greenland (or at least close enough to pick up each other's signals), a sufficiently well built receiver could detect American radio transmissions, and there is also the option for visual recon prior to landing.