If there is active fighting over the city, whoever controls the main train station (or, if destroyed, the piece of ground where the train station used to be) is counted as controlling the city.

This is question #2 in the Astral Codex Ten 2023 Prediction Contest. The contest rules and full list of questions are available here. Market will resolve according to Scott Alexander’s judgment, as given through future posts on Astral Codex Ten.