Subway system

METRO

["Think of the underground trains commonly found in major cities like Paris or Tokyo, often marked with a distinct \"M\" symbol."]

"Subway system" is a straightforward clue pointing to "METRO," the common term for urban underground or rapid transit networks in many cities worldwide. The word metro comes from metropolitan railway, and while it can refer to any city-based train system, it’s often used interchangeably with subway—especially in places like Paris (the Métro) or Washington, D.C. (the Metro). No wordplay here; it’s a direct definition where the answer fits naturally.