Of course you're assuming that TSA won't be involved with rail travel if it became more popular.
The primary impetus for modern TSA was a group that figured out modern jets were useful weapons if diverted off course. While train diversions are still dangerous and catastrophes, they are nowhere near the same level of useful weapon in the hands of terrorists.
The TSA does theoretically screen for bad actors and explosives, both of which are still relevant to rail travel, even if they're potentially less threatening there.