in my experience, a person's train of thought is two basic things. One, the data they receive. This includes internal and external data. Two, their means of interpreting that data: integrating it, associating it with other data, providing themselves an avenue to experience it, basically. This is an oversimplification, but whatever.
as a separate person, it's completely possible to observe at least a part of what data someone is receiving, merely by knowing what stimuli exists, or what stimuli may exist. Then, you look for clues which either confirm or deny your understanding of how that data is interpreted, and build a better picture. With a good enough picture, you can even begin predicting mental stimuli which arises independently, despite it having no immediate real world indication, just by understanding its context. Then, you can gauge your own accuracy by how this stimuli incites further action in the observable world, matching that action up with your understanding of the mnemonic and associative cues native to whoever is performing it. Does such and such train of thought lead to x, y, z action? Etc. etc.....
I don't experience peoples' entire trains of thought by doing this. I experience individual factors which contribute to trains of thought, and then, if they are coherent, knowing the train of thought naturally follows. It might look paranormal to somebody who has no clue what it is, but once you have an understanding of how the whole thing works, it's just a matter of filling in blanks and is incredibly simple.
I've been doing this reliably and verifiably for something like three or four years.