Given the fact that Amtrak didn't even allow unloaded firearms in baggage until Congress forced them to, I find it very unlikely that they would allow concealed weapons in the passenger compartment.

They are a unique private company, but a private company nonetheless, so they can pretty much make whatever rules they want. That is why Congress cannot actually tell them what to do, but only threaten to withhold funding if they don't do it. I don't know what their rules are, but remember that if you are found violating their rules but not breaking any laws, then in addition to making you leave, they can ban you from Amtrak trains in the future. If I were you, I would not carry a concealed weapon on an Amtrak train unless you have a statement from Amtrak in writing stating that it is permissible. Don't rely on what some employee says, because you have no proof, and even if you do, they can always say that the employee gave you wrong information, etc.