As I recall, and it has been a long time, when Amtrak was created the railroads said that they would not let anyone else run passenger service on their railroads unless they were free of all liability. If that had not been agreed to there would be no long distance passenger trains today in the US. When Amtrak was started most, if not all, long distance passenger trains were loosing money. This was impacting other railroad operations and the ICC had been approving passenger train discontinuance at a rapid rate. This was a result of the USPS adopting a hub and spoke distribution system that put and end to mail by rail, which was supported passenger train operation.
It was not a secret. The railroads did not want to face large liabilities for operations they had little control over. In trade for Amtrak assuming this liability, they did not have to pay the railroads for assuming it and insuring against it.
