Subway systems are an underground network of subway tunnels and subway stations that allow for the transport of people and goods beneath a city, building complex, or other settlement. In fictional worlds with dense metropolises or surfaces that are exposed to extreme conditions like storms or the vacuum of space, subway systems may be a preferred means of transport. They are frequent elements in post-apocalyptic worlds where they may serve as an additional means of navigating a ruined city but may hide dangers of their own. Subway systems may also be repurposed as bunkers or safe havens from above-ground risks. They may contain abandoned stations or segments of track.

When designing subway systems consider the areas that need to be linked together. Locations where many people must travel to (work and home) are likely to have subway stations. As will sporting arenas, transportation hubs, city centers, parks, and other popular or central locations. Also consider the fee that must be paid in order to travel and how people are kept from riding for free. Subway stations may have guards, ticket-checkers, gates, or other means to ensure payment has been made.

The design of the train cars themselves should also be considered. Different cars may have different passenger capacities. They may look different with different styles. Train cars that service the slums may feature graffiti. Subway cars may also be a location where advertisements are on display and worldbuilders can use them as an opportunity to showcase fictional companies and products.

Creating a subway map may be useful.