The narrator here is wholly omniscient, meaning that the narrator can let us in on any character's inner thoughts or back story at will. But we rarely hear just the narrator's voice, independent of any character. The majority of the narrative is filtered through the point of view of various characters, and this point of view rotates continually.
For the most part we stick with the filtering views of a number of major characters, and the character focus shifts during the various scenes that make up larger chapters. These focus shifts alter the narrative filter accordingly, meaning that in Hank's scenes we hear his thoughts, in Dagny's scenes we hear her thoughts, and in their scenes together we ping-pong between their thoughts.
The rare occasions where the narrator butts in are notable. The narrator takes us through the crash of the Taggart Tunnel without filtering through a major character, which helps to emphasize the huge impact and scope of the disaster. We get coverage of a large number of people and events instead of any one person's detailed inner thoughts.
Another notable example where the narrator takes over is during the scene leading up Cherryl's suicide. The narrator actually comes out and says that he/she is translating Cherryl's final thoughts to us, since Cherryl was too stressed to be articulate. In this case, the narrator helps highlight the tragedy of Cherryl's death.
Overall, though, the narrator mostly stays out of the way and simply presents a revolving door of character perspectives to us.