Again: I'm not saying he's a heroin dealer, and I said that specific to avoid this pointless preening about how it's everyone's right to carry large amounts of cash on them. Sure, I'm fine with that; in fact: I think you will find it difficult to find anyone to take the other side of that argument on HN (and HN is a big, complicated place, giving you some idea of just how banal that argument is.) So let's assume that's not what I'm talking about.
If it helps, though this isn't really my intent, assume my subtextual allegation is that this story is copypasta.
I used to play poker semi-professionally. Traveling with upwards of $50k wasn't an uncommon thing at all among the people I knew.
I knew a card counter who was detained at an airport carrying well in excess of $100k and, comically, the money and the inside of his bag were covered in white powder at the time. Because he was a nerdy jewish kid, they believed his story that it was powdered caffeine (it actually was) and sent him on his way.
Carrying a ton of cash is unusual, but does really happen.
When receiving a large amount of cash for a legitimate sale of something, or other kind of legit transaction taking place, the first thing most people would probably think to do is to go to their bank branch in the local area and deposit the money ASAP, and then get on the train. Large deposits are tracked, I think you have to fill out a form, so maybe he just didn't want to have the tax man look at what's going on (points to possibly shady dealings).
The fact is that walking around with $80,000 in cash is a huge risk (unless you're a billionaire - but then you're probably not taking a train), it's not something most people would typically do. It appears a bit shady without OP giving any other details. He claims it wasn't for heroin, but what if he's purposely omitting that it was actually for cocaine. I'd like to know more about OP's story, but I doubt we'll get any answers.
Shall we send them over to your place next, so they can tell you what exactly you have to hide?
The fact is, if you want to keep your money safe, don't walk around with large sums of it. Period. It's really that simple.
The money didn't magically appear in his pockets.
Correct me if I'm wrong, but this very much sounds like "drugs". If that's the case, your point about him never being involved in "heroin" sales is now just semantics, because he's still engaged, to some degree, in the movement of drugs between states.
I'm thinking they saw a young guy on a cross country trip who had purchased a ticket on short notice and decided to go after him. Maybe he was "acting suspicious" if he was nervous, but I kind of doubt that; he used to take beta blockers for that type of thing and what he was doing wasn't really out of the ordinary for him. Maybe they saw he had a history of taking the train that route. I don't know. But I do know that they lied in their justification for making contact with him.
Obviously, it isn't.