trains are a boring enough way to get across america. I would advise renting a car. americanroadtrips is a decent site. you have a couple of options, preferably renting in one airport and dropping off at another, or buying a crap car and getting insurance etc....if only for 2 weeks though i would advise just rent a car.
It depends what you are looking for really, nigthlife? things to see etc or what? i would try and start in chicago and work the way across the north, reserves and great things to see along the way, bar possibly nebraska and the corn fields of the mid-west. Washington is a great city for museums and stuff, very clean, but lacking in anything else really. Boston and New York are good, but if only there for two weeks and driving i wouldn't start there.....
San fran is good all the way up the west coast is good really, definitely offers something different, and you certainly "feel" like you are in a different country a far way away. I amnt really saying much here, but if you give more detail on what you are looking for then it might help!!
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