Just got back from my first trip in coach..LA to Chicago and back. I felt very safe w/ my belongings left above the seat while in the lounge car where I spent much of my trip. Like others said, I would not leave computer or cell phone out but I think the whole environment felt very safe.
I guess I'm pretty cavalier about this, but I leave my clothing/larger bags in the overhead area for the whole trip, and often leave a shoulder/book bag at my feet when moving about. My camera lives in a belt pouch (I'll be using it throughout the train, in the lounge or in the diner) and my phone lives in my pocket, but my laptop and laptop/phone chargers, yes, they tend to stay in coach.
My policy is "make friends with my seatmate early, and assess their character"; and I think I've only had a couple seatmates of dubious character in years of travel. Once I have a sense that my seatmate is unlikely to do anything malicious (and the knowledge that they're traveling some distance--eg, I'll be back in my seat before they're off the train), I'm comfortable leaving my things there, assuming they'll keep anyone else from taking my bag (and knowing I can casually make sure my laptop and chargers are still there later).
I've also got a general belief that a group of familiar strangers polices itself--your neighbors don't change very often on a train, and even though most of you don't actually know each other, you become familiar, and odd activity is likely to be noticed.
And "make friends with my seatmate" has all sorts of other benefits! Great conversations get started that way--that's one of the marvelous things about train travel, especially long-distance train travel. And perhaps I'll write more about one of those in MStrain's thread later...