[replying to initial post, not having read all subsequent posts...]
First story: Late 2009 A friend and I hopped a grainer out of Sparks, NV and ended up in Elko. (skip the beautiful scenic details) We were spotted by some rail workers as we hopped off in the morning. We hitched into town and later that day were accosted by the local five-o. We were pretty much reprimanded as if they knew we were the guys. They didn't have evidence, so they couldn't arrest us--which they surely would have done--but they did tell us to get the hell outta town.
Second story: Trying to hop outta Elko the next day, my friend and I spent a good eight-or-more hours stuck inside a grainer. A rail crew doing maintenance on the car adjoining ours unknowingly had us trapped. They didn't see us, but, man, were we afraid to come out. I imagine, merely for safety's sake, they would have ejected us from our settlement and, merely for propriety, they would have pressed charges.
Third story: The train later took off with us inside. (skip the beautiful scenic details) As we came to SLC, we actually chose not to get off. If I remember correctly, we had the idea that the yard would be too difficult to escape without being caught. We feared the illegality of the situation.
Fourth story: (skip the beautiful scenic details) We ended up in Western Wyoming... where some time later we chose to hop out again. The night in question, as our train pulled up, we spotted a(n infamous!) pusher. We weighed the risks (taking an intermodal or a grainer over a pusher--THROUGH THE SNOW??) and ran for the pusher. We knew we wouldn't be spotted, as the front of the train was well ahead and out of sight; but we did fear that the train would pull into the yard and stop, and that a rail-man would enter the pusher and thus find us and incarcerate us. Suffice it to say that youth yields one to an astonishing bravery. We took the pusher and passed through the yard unmolested. From there to about midway to Cheyenne, we were fine. The snow was reflecting wonderfully the moonlight, yadda yadda yadda... It was freezing cold outside. This fact makes the next part somewhat understandable. Amidst a hilly area, where no city lights could be discerned in the distance, our train stopped. Minutes later, a second train was seen approaching on our track. Minutes later, it touched ours. Since the engine's nose was facing backwards, it connected perfectly with the forward-facing nose of the new train. Minutes later, much to the surprise of my friend and myself, a conductor waltzed right in. He was a gray-haired man, long in years of experience. Well, I'll be damned.. all he did was say hi. Nonplussed, we said hi back. He laughed a bit and told us (after telling us that he "understood our situation") not to touch anything or mess with any papers. If we agreed to comply, he was sure not to have seen a thing. Before parting, he showed us the mini-fridge (which we had already found, haha) and gave us each a bottle of water.
Last story: Months later, with a different friend, I caught a train out of Houston. We spent a near-fatal night pushing through wind and rain, lying exposed on the backside of a grainer with no dog-holes to sleep in. We pulled into an area near Lottie, LA in the morning. We decided to ditch the train and started walking towards the front engine, where we thought we saw a roadway. As we got to the few cars before the engine, we got nervous. Well, I walked right up to the thing and spotted the window opened. I yelled up for a bottle of water, not knowing what else to do to get by... AND GOT TWO! Now, conductors aren't wholly moronic. When they see a guy with a pack the size of a large suitcase on his shoulders, he knows he's been on his train. Well, if ever there was a time for that man to do something, it would have been then. But he didn't. Period.
So... how illegal is hopping? It depends on who you are, where you are, and who you run into. Being amiable can mollify a rail-yard bull; being caught in the middle of nowhere can make a conductor rethink kicking you off; being young wins hearts. You see what I mean? It's only as illegal as the guy makes it who catches you. And remember:
It ain't illegal if you don't get caught. (don't quote me on that)
Sauve qui peut.
May he save himself who can.