The Call of Duty League appears to have leaked a new update to the Call of Duty: Warzone map coming soon.
Per their “congratulations” GIF asset for the Dallas Empire winning the CDL Champs event, the bottom left of that showcases a Subway station near Stadium that does not currently exist in the game.
In addition, @geekypastimes showed another image from the broadcast which gives a better view at the new addition:
This new Subway station feature is expected in Call of Duty: Warzone when Season 6 starts, which is expected in late Sept. or early October.