I think you can figure out the difficulties from that.
ShadowAce, you bring up a very good point. All trains must run on track. To save money during the building of track, most of the track in the United States is single track. Since it is privately owned, it is like a private road. Private companies use the track for their own trains, and charge other freight companies to use their rail lines.
To make the train transportation market similar to the air carrier market, you would need to nationalize all of the physical rail, many of the rail stations, and then create a national rail managment center to direct traffic. Considering that there are some rail lines / corridors that are already heavily used, you would need to further invest the building of additional rail lines / bridges / corridors / spurs
All of this adds up to a very expensive proposition.
Yes that is the turd in the punch bowl everyone ignores.
And the only way to go about fixing such is to have the Gub'ment build more rail lines and buy up the ones already laid and have them administered by the FEDGOV which the mere thought of makes me nauseous.