I feel it's a hybrid, truly. There's scope for exploration, but the nature of it is more like a 'wide open corridor' than a wide open world. I mean, I remember trying to access caches in a train graveyard at 3am, just wandering about the world trying to scavenge un-noticed, which is not something you can do in a more conventional 'on rails' shooter. On the other hand, once you've done the series of side missions and get on with the plot, you just push forward. There's very much a sense of progressing through the story, whereas in the concept of an open world game, like say the X series of games, you can just drop it. Fly off into space, become a pirate or an industrialist, etc. You don't need to ever advance the plot, but you still get a fleshed out game. So yeah. Hybrid, for sures.