I get the impression that some law still expressly allows (though with limits) lawsuits by and settlements with passengers. Otherwise, by now, Amtrak, the bus lines, and the airlines would all have moved to the standard corporate "individual arbitration of disputes only - no lawsuits of any kind" provision in contracts for carriage or service.

Is or is not Amtrak a federal government agency? If it is, then perhaps some type of governmental immunity might apply. But we know that it's not: it is a government-controlled corporation, but it is not a government agency. Several courts have found as such, and the laws setting it up expressly did it that way. So the laws about suing a corporation clearly apply.