Did you ever use the tank or dodo plane in Grand Theft Auto 3 to fly to the ghost town that was used in the opening cutscene? How about glitching out of any map in Halo 2 and exploring? Much like the latter case, a player has found a way to break out of bounds on the train section and explore. The catch is, there appears to be a fair amount of unused assets laying out in the wild.
Three ideas have come up regarding this discovery: it's unfinished assets whose completion was abandoned before launch, it's just background construction you see from a distance while on the train, or that its related somehow to future DLC.
Whatever it is, I always find it both fun and terrifying exploring out of bounds on games. It's when truly anything can happen.