i've never encountered any security whatsoever travelling by train, even on long trips (like 8 hours). this was in canada, but a few of the routes sometimes are serviced by Amtrak trains (and i'm pretty sure that Amtrak is the only US passenger train service).
they never did anything more than ask for my ticket -- except once, on an Amtrak train where they asked me if i understood how to use the emergency exit window, and told me to move to another seat if i didn't think i could handle the responsibility of sitting next to it.
oh, and the amtrak trains have food on them. food and alcohol.
but no smoking. keep this in mind. no smoking. pain.
however, all this was before sept. 11. they may have increased security since then.