Part of the problem is distance.
The distances in the US are too great for rail to be competitive over air.
Another is timetabling.
When I travelled on the Texas Eagle from St Louis to Dallas, the train passed through Lille Rock, Arkansas. There is only one train per day in each direction at Little Rock, and the Southbound is supposed to depart at 03:15. Similarly for the Northbound train, which leaves in the small hours of the morning.
When I was on it, it was two hours late.
So if you lived in Little Rock and wanted to go to St Louis, Chicago, Dallas or Houston, are you going to hang around a railway station in the middle of the night, or find another means of transport.
The other is the level of service.
I also travelled Chicago to St Louis. It is 260 miles, there are only 5 trains per day, and the journey takes 5 hours 30 minutes, if on time.
Compare that with the level of service from London to Newcastle (247 miles)
The State of Illinois is investing in "high speed" (110mph) rail, but for a lot of the time there is only single track on the Chicago to St Louis line, and freight trains are always given priority.
I don't think the new president of the US is going to help matters either.