As mentioned above AM reception can be spotty, however if you're looking for news or other 'spoken word' programming such as weather and sports reports or community announcements, AM radio would probably have more of that type of programming than FM which is primarily music with shorter range signals (although strong FM signals can sometimes probably penetrate train cars better and are less susceptible to noise).
But for information programming the higher-powered 50,000 watt AM stations would provide the best listenable signals on the train, plus they are wide coverage stations, especially at night, enabling you to stay with a station longer without constant retuning. Also stations lower on the dial often have very good coverage even if they're lower power.
Some of the high power stations along the Texas Eagle and Sunset Limited would include AM 670, 720, 780, and 890 in Chicago, AM 1120 and 550 St. Louis, AM 1090 Little Rock, AM 1080 Dallas, AM 820 Forth Worth, AM 1200 San Antonio, AM 600, AM 620 Phoenix, and AM 640 and 1070 in Los Angeles.