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.
/me updates mental notes
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.
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.