I think I'm the opposite of everyone else here--I enjoy the challenge of cutting it as close as possible. Knock on wood, I've never missed a train or plane.
This morning, I arrived at Denver Union Station all of 20 minutes before the train left. 20 seconds to enter the station from the taxi drop off out front, 20 seconds to walk over to the Quik-Trak machine, 60 seconds to locate my reservation and print my tickets, and then 10 minutes wasted waiting in line for the conductor to take my ticket and issue me my seat check (interesting, he did it from a podium inside the station rather than on the train--never seen that anywhere else).
Oh, all right, it wasn't completely wasted--I did spend the first five minutes enjoying the architecture of the station's interior.
From an unstaffed station that's nothing more than a single platform (my two main stations, SIM and PRB, are both about as plain as you can get), I wouldn't be uncomfortable until the 5-minute mark (I usually arrive somewhere between 5 and 10 minutes before departure, on the higher end if I'm driving and will have to park and on the lower end if someone's dropping me off). I'd add a couple minutes if I have to pick up a ticket from a Quik-Trak machine (more if there is none and I need to go to a ticket agent--that would necessitate me arriving more like 15-20 minutes ahead--a good example of this type of simple station without a QT machine would be CLT or CVS). And last, for big stations I'm familiar with, I usually plan to arrive 20-30 minutes ahead of departure--maybe more if it's a terminal (e.g. LAX) and I want to try and get a good seat. I once arrived at LAX with barely more than five minutes to make my Thruway bus and still found a parking spot and made it--an experience I'll probably not intentionally repeat, but the adrenaline rush woke me up far better than coffee could have...
I used to love going down to the SLO Amtrak station as a kid and run up and down the outside of the Amtrak trains and see the SP (and shortly thereafter, UP) freights passing through. I still enjoy trainwatching if I'm somewhere and I happen to see one go by (and sometimes I'll chase it for a few miles if I have nothing better to do), but I rarely go out of my way to seek them out. I don't like sitting and waiting--it feels like I'm doing nothing and not redeeming my time well--life's too short to waste time. Maybe that's why I hate arriving too early for trains and planes, too--it feels like wasted time (I even get antsy once I've boarded the train and we haven't left the station yet--come on, people, I want to MOVE!). It might also explain why I dislike purely beach vacations--a few minutes after I sit in my beach chair or wade into the water, I start to feel antsy that I'm not actually doing anything and seeing this huge wide world. Why sit on the beach in Kona when there are volcanoes and lava tubes to explore and geographical oddities (like South Point) to visit and say I've been to!
In summary, at an unstaffed, simple station like Barstow, I think if you planned on arriving at the station 10-15 minutes early, you'll be perfectly fine. That gives you an extra couple minutes to unload your luggage and walk over to the platform (rather than sprint like I do). IMHO, 30 minutes is completely unnecessary.