The Ascent is one big interconnecting world. You can get to new areas by taking elevators and the like, but each floor is segmented into multiple areas that you can explore. But some of these areas are filled with higher-leveled enemies that you do not want to go into, as death will be assured.
To get back and forth between these areas and the central hub area Cluster 13, there is a fast travel system. Two, in fact, and both are completely optional. If you want, walk everywhere. You'll have more of a chance of finding more hidden chests or bounties to complete.
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The first is you can catch a train for free. There is a train station in most areas that is symbolized by a Red M. All you have to do is go to one of these red m's and wait. It may seem like nothing is happening, but just be patient. A train will eventually arrive. Sometimes you'll even find underground stations. Then you can select any destination that has been unlocked, aka you must have seen the train station.
The second way is that you can call a taxi no matter where you are, it costs Ucred, the currency in The Ascent. To do this, you need to complete three-quarters of the mission Trading Places, where you have to locate Coders' Cave and speak with Noghead. Upon leaving Coders' Cave, this taxi fast travel mechanic will unlock. It will cost you 1000 Ucred every time you use it.
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