If you wanted to make a faithful recreation of that scene… well, Im not sure that’d be possible in Hammer. Or at least not practical. It’d probably involve a lot experience and a huge scripted scene, or a lot of func_trains. And you’d need a huge skybox, or a relatively big one that’s a train itself that keeps going back to start. And even if you pulled that off, I’d assume it’d lag a lot. Or maybe there’s a better way to do it, idk.
However if you’re going to be simplifying it a lot, Id go with func_trains like coobydoo said or a scripted scene. Use either a custom model or a smoke effect for the clouds. Although I think you may still run into the big skybox problem. Anyway, that’s just what I have to say. Im not exactly a great mapper.