For a lot of trips, Amtrak is about the same as flying. Looking at a round trip from NYC to Chicago, on the arbitrary dates of March 26 to April 9, Amtrak costs $180 while Kayak shows the cheapest flights at $159. Of course, Amtrak takes much longer (20 hours) but is less hassle, and at that price the flight only lets you take one carry-on while Amtrak lets you bring two checked bags and two carry-ons. Adding checked bags will add $60 for the first and $80 for the second. Amtrak is also better for the environment if that matters to you, and the price difference is about the same as the cost of the carbon offset for the flight.

Overall, choosing Amtrak or flying is perfectly reasonable in different situations. Amtrak could definitely be improved a lot though!

NYC to Chicago probably has a decent customer base to support that price point. For amusement, try Miami/Seattle via Amtrak. It's possible, since I looked up the route [1]. One week transit time one way, at $2,000 (I don't recall if the price was round trip or just one way). And trying to work on the train was fun (too "swishy" to use the computer: http://boston.conman.org/2015/08/05.2). Totally not worth it for me. Sadly [2].

[1] For reference, I live near Miami (technically, I live in Boca Raton, but Miami is "close enough" at this scale). The company I work for is based in Seattle. I refuse to fly anymore for a variety of reasons.

[2] Riding in a private rail car really spoiled travel for me: http://boston.conman.org/2015/08/05.4 But way out of my pay grade.

I don't think I kept it, but a couple of years ago I made a video of a £1 coin (as thick as two US pennies) balanced in a shallow groove in the table on a long distance train in Denmark. It was about 30km before it fell over.

(But maintaining the track to this quality would be even more expensive if there was as much freight on it as tracks in the USA carry.)