When two teams from the same city face each other in the World Series or another series of games in interleague play, and that city has a subway system, the Series is then designated a subway series, with the idea that the players and fans could take a subway ride from one stadium to the other to watch the games.
Baseball has traditionally offered only three possibilities for subway series: New York, Chicago and Oakland/San Francisco. As teams move and more subways are built, the possibilities for such series will also expand. Major League Baseball had 14 such World Series in its first 140 years of existence, always involving New York teams, who are thought of immediately when the term is used to refer either to the championship series or regular season series. A subway series is easily accomplished in New York because both Yankee Stadium and various other stadiums of the Yankees' opponents (Ebbets Field, Shea Stadium, Citi Field, among them) have been easily reachable by subway in The Big Apple.