Though the link does not directly address the access question, it has lots of interesting background and footnotes to the relevant acts. I herewith freely quote from the link and it footnotes, which are primarily from the 1970 act that eventually formed Amtrak, as I insert [comments and clarifications] in square brackets. Note the last paragraph…
“From the middle of the 19th century, the railroad passenger coach played a significant and sometimes romantic role in American cultural and economic life. By the middle of this century, however, ‘this time-honored vehicle’ threatened to take its place in the transportation museum along with the stagecoach, the sidewheeler, and the steam locomotive. Whereas in 1929 about 20,000 intercity trains operated in the country, by 1946, there were only about 11,000 such passenger trains; by 1971, fewer than 500 passenger trains still operated. As cars, buses, and airplanes displaced the passenger railroads, those railroads that continued to provide passenger carriage incurred heavy and continuing losses. At the same time, as common carriers these railroads were bound to continue providing service until the Interstate Commerce Commission (ICC) or state regulatory authorities relieved them of this responsibility. Given the tremendous operating losses, many of the remaining handful of railroads operating passenger coaches sought ICC permission to discontinue passenger train service.
“The Rail Passenger Service Act of 1970 … was enacted in an attempt to revive the failing intercity passenger train industry. For this purpose the Act established the National Railroad Passenger Corporation (Amtrak), a private, for-profit corporation, authorized to operate, or contract with private railroads for the operation of, intercity rail passenger service. [Profit was allowed but not required in the 1970 Act.] Most private railroads offering such service entered into ‘Basic Agreements’ with Amtrak, and thereby, as provided by the Act, shed their intercity rail passenger obligations.
“Although there are numerous commuter rail operators in the United States, Amtrak is the sole provider of intercity passenger rail transportation. Formally known as the National Railroad Passenger Corporation, Amtrak was created by the Rail Passenger Service Act of 1970. Prior to Amtrak’s creation, passenger service was provided by the nation’s freight railroads. In return for government permission to exit the passenger business, freight railroads donated intercity rail passenger equipment to Amtrak and helped capitalize Amtrak with significant cash payments.
“With the prospect of relief from the hundreds of millions of dollars in annual losses associated with passenger operations, freight railroads agreed to awarding Amtrak guaranteed priority access to the privately owned U.S. rail network, with Amtrak’s access fee limited to the incremental costs (e.g., track maintenance, administration, and emergency services) that freight railroads incur for passenger rail service.
