This is the comment I wrote almost a year ago: https://news.ycombinator.com/item?id=6870933
I was not aware the USA has so many information about every single travel. According to this [1] they record everything.
- amtrack
- trains in the EU (including sit numbers)
- bus travel in the EU
- plane travel in the EU on a carrier that doesn't fly to the US
- hotel reservations
- pedestrian border crossing
Everything.
[1] http://hasbrouck.org/articles/Hasbrouck-Cato-2APR2013.pdf