The US freight rail infrastructure is in good shape. In fact, I'd say it's probably in the best shape of any common-carrier freight rail network anywhere in the world.
Our passenger rail network, however, is a sorry mess. The Northeast Corridor, owned mostly by Amtrak but also by commuter rail agencies in New York and Connecticut, is plagued by over half a century of deferred maintenance and lack of upgrades first caused by the freight railroads in the area (Principally the old Pennsylvania Railroad and the old New Haven Railroad) trying to stave off bankruptcy, then by the grossly inadequate funding of Amtrak caused by the Republicans' privatization fetish. As a result, we here in the US have a passenger rail network that would deserve to be laughed at in derision by the wealthier countries of northern, western, and Central Europe and by Japan.
NJ Republican Governor Chris Christie's recent cancellation of new proposed trans-Hudson tunnels for Amtrak and NJ's commuter agency are just one sorry example of Republican negligence.
Passenger rail outside the Northeastern US is often in equally sorry shape. Aside from the fact that I've come to believe that passenger rail traffic should run on separate tracks (if not necessarily always on separate rights-of-way) as often as possible, certain segments of Amtrak's passenger rail network are long overdue for repairs and upgrading. I know little of the Chicago to Detroit line owned by Amtrak, but I do know that the route of Amtrak's Southwest Chief through Colorado and northeastern New Mexico is sorely in need of new rail and repairs, and the lack of a through rail service through Phoenix AZ is a scandal.
Amtrak is in sore need of new cars and new locomotives so it can not only handle increased traffic on the trains it already has, but also to open new routes and increase frequencies.
I wish I could offer a prescription that would fix everything, but I don't. I do know that the overwhelming majority of Republican Party elected officials are stuck in a mind-set that mirrors that of oligarchical Latin American countries half a century ago. Replacing them with Democrats would be a good start, but ONLY a good start for what ought to be done.