I have heard that there is also a secret level as a homage to id Software's game Doom, in their game RAGE.
Where can I find this level and what do I need to do in order to access it?
To find the Doom level, you will need to travel to Gearhead Vault, within Subway Town. Once you're able to access this level;
Here is a youtube video detailing how to find this hidden room:
Finding this secret level will also allow you to find the Doom Marine bobblehead item: