The game just doesnt give you enough instructions on why it isnt usable. Sure you can take the train to begin with, but if you took a look at the tutorials youll see that you can actually fast travel around the map via taxi by simply pressing T on your keyboard. But no matter how much you press it, the taxi never actually shows up. Why is that? Dont worry, its not a glitch.

Source: N4G PC The Ascent Guide: How To Use The Taxi