Large amounts of cash, and not even all that large ($10k) have been considered evidence of crime and confiscated by the cops in the US. The laws that enable this have been slowly dying off, but it still happens. No other evidence of a crime is necessary nor do the people need to be convicted of anything, the cops just take the money because it might be used to commit a crime.

http://gm1-miwebvarnish.newscyclecloud.com/opinion/20201218/...

even on trains? amtrak specifically questions people about cash?

This seems weird -- compared to traveling by air amtrak has approx 0 security, no TSA equivalent, bag scan, etc(unless you check maybe).

They check cargo holds and do random bag checks - as of the mid 2000s due to the "war on terrah" but maybe that went out of fashion in the past decade. Large denomination bills are designed to be easily scanned externally since they contain metal strips in the bill design.