Sure, people carry around cash all the time, there isn't a problem with that - but carrying around $80,000 in cash is highly unusual. It's not something normally done. I don't know anyone who would travel with that kind of cash because it could be stolen, by cops or by anyone.

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.

The fact that it's unusual is not relevant. Or it's relevant in that - pretty much by definition - these situations are unusual. It's not okay to violate people's rights merely because "unusual". It's not okay to demand a good story just because "unusual". Instead we get the all too common a reasoning "well, I don't play cards", or "well I have nothing to hide". None of this is relevant. The opposite is true: if you believe in freedom of speech for another example, then it's only for socially uncommon speech that it will matter. In particular, speech that few people would support. If cash is legal, it's only in unusual situation that it has to be particularly protected by the law.

Hanging out on HN and arguing in favor of the gestapo is unusual.

Shall we send them over to your place next, so they can tell you what exactly you have to hide?