Did the parent (s) buy the underage passenger her ticket with their credit card ??? If so, it would seem to me that the parent(s) are accepting responsibility for her safe travel, within Amtrak's guidelines. I agree that they should have researched Amtrak's Conditions of Travel for Minors. I wonder if the girl tried to contact her parents via cell phone; perhaps a brief conversation with the conductor would have solved the problem. Sounds like a money deal ( wrong class of ticket bought )to me.
In August of 1979, at the age of 15, I rode The North Coast Hiawatha from Billings, MT., to Seattle, WA. I then managed to find The Coast Starlight without any help and ride it to Los Angeles, where I again found the correct San Diegan train to finish my journey to Fullerton, Ca. I arrived in Fullerton on my 16th birthday. That's 3 days and 2 nights alone, without the supervision of a parent or guardian, in a coach. The only problems I encountered, were a lack of meal money by the 3rd day, and a conductor on the Starlight who did not like my habit of riding in the vestibule with the upper dutch door open !!! Times have certainly changed...