id Software states that you must buy a new Rage if you want to use the sewers.
A number of publishers are cutting off game content with things like online passes (one-time use code that be generally unavailable to those who buy any game used). Usually this cuts off things like online multiplayer or highly praised game content. In this case, Rage won't.
From this article on Kotaku and in an interview with Eurogamer, Tim Willits, id Software's creative director, stated that access to the gamer's underground will only be for those who get their copy of Rage at retail and as a new purchase.
Tim Willits stated the following:
"You've played Rage five times. Have you found any sewer hatches? If you bought the game new, that would be open for you...you still have to download it, but you don't have to pay for it. Those hatches are all over. Most people never find them. But as soon as you do, you're like, oh. And then you start to look for it. That's our first-time buyer incentive."
Willits believes this method is much less obnoxious than the usage of online passes due to that the fact that people will know what they are missing but with Rage, they will not.
For those of you who don't know, Rage's sewer-system content is, in a way, a "outside the main path" game content. So meaning while it is absent to those who didn't buy the game new, it won't diminish the story or experience.
No news if use of sewer access will be made available later via a separate DLC unlock.