Edit: the rail pass, naturally. But it's a lot more restricted: "Passes are available in three travel durations and travel segments (15 days/8 segments, 30 days/12 segments and 45 days/18 segments) through out the entire United States. Travel must begin within 180 days of the date the pass purchased." ...and there are some other restrictions.
It also isn't noticeably cheaper: US$389 for the 15-day pass, US$549 for the 30-day pass, and US$749 for the 45-day pass.
I did this alone for my winter break of freshman year of college. To the best of my recollection, my itinerary was Portland -> Vancouver (BC) -> Toronto -> New York -> New Orleans -> Ft Worth -> Joshua Tree -> LA -> SF -> Portland. One month. I arranged to either sleep on the train or crash with friends (knew someone in most of those places). The diner car was pretty expensive, so I generally carried a little food and ate heavy when I hit stopping cities. A significant portion of the 30 days was spent in a train, from what I recall.
Traveling by train has a special extra something; you're with the people you travel with for long enough to actually develop a conversation, but you're with them just short enough that you don't have to worry too much if you don't hit it off. This was great for an introvert like me to practice chatting up strangers with little risk in a safe setting. And you meet a really interesting mix of folks, different from the people I normally find myself talking to. It's particularly fun to feel the gradient of humanity / culture shift as you travel.
I traveled light; just a backpack, a camera, and a laptop (well, and climbing shoes for J-Tree). I was super careful about keeping the tech gear hidden and slept wrapped around my bag when I slept in public spaces (train stations / trains).
One thing about traveling by train: Amtrak is always late, and often by half a day. Make sure people who might be meeting you at connections know this and don't expect it to work like air travel.
Anyway, my thirty days of independent travel was a fantastic experience, and I'd really recommend it to anyone, young or old. You can even comfortably get work done in the passenger cars. Tether to your 3G phone or whatever and you're riding the Rails in style.
Yeah the Amtrak passes suffer from the problem of too much complexity, which makes them risky for the buyer. The restriction to only 4 trips on any given segment is especially killer. Their own example shows that going from DC->NY and back would use up 2 trips, then DC to Philadelphia would count as trips 3 and 4 because it happens to travel the same segment of track for a while as the DC->NY route. That is way too hard for the buyer to figure out on his own.
I wish they still offered the unlimited riding for 30 days passes that they had several years back. Eurail passes are immensely popular, partly because they are easy to understand.
What I was in Boston last summer, I took the train a lot to visit various cities on the east coast. The train turned out to be a really great place to hack because they had power outlets, a cafe, lots of seating space (with tables), and, most importantly, no distractions.
Amtrack now has designated "quiet cars" for people who need to work or read. No ringing allowed, and when I rode recently, plenty of seats with tables and power outlets.
