Subway

You've always wanted to walk down the steps you see so many people walking every day. You walk down and realize it's a lot different than you thought, colder, dirtier, smellier, and louder. You feel a gust of wind and suddenly a train speeds in next to people waiting to file in and out of the train. You see two different signs pointing opposite directions. The sign pointing right has a green number 6 on it. The one pointing left has a blue letter E on it.

Image source: PrecipiceofDuck,"Entrance to the station at the Lexington Avenue and 53rd Street plaza", Wikipedia, https://creativecommons.org/licenses/by-sa/4.0/