Agree strongly with the recommendation to get a notarized letter signed by both parents; be sure it describes the dates and trip you will be taking. Have it also authorize emergency medical care needed during the trip. That's what we do when we travel with our granddaughter.
But, having gotten the legalities out of the way, one question to ask is whether the 8 year old in question is comfortable being around you without his parents being there. It's one thing to have an exciting train adventure with Mom and Dad around, perhaps another thing entirely if they aren't there. This isn't at all the same as being an unaccompanied minor on a plane, where the flight attendants buffer the experience pretty well, and it's likely much shorter than the bedroom train trip in question. I'm assuming, though, that his parents will have a good sense of whether this is in his comfort zone. If they say yes, you're fine. If not, don't take it personally--he may just not be quite ready.
Having traveled on trains with our special needs nephew as a boy, I will suggest that kids can get bored on a train, even those that start out excited about being on a train. We generally don't believe that a lot of screen time is great for kids, but on a train, I'd have some kids games apps loaded on a tablet or phone for those "When are we gonna get there?" times. Drawing pictures, playing with Playdoh, favorite books, will all pass the time. A book about trains might be fun. Again, I don't make a habit of plying kids with sweets and treats, but, assuming Mom and Dad agree, a bag of lots of different kinds of goodies doled out judiciously can dispel boredom, too.
When our nephew was seven, we took the Coast Starlight from Seattle to Los Angeles with him, and today at 11 he still talks about it. If he's ready (and you're prepared, too!) this could be a memorable experience for you both.
