A subway is not technically a train, but the tunnel and track where the subway train runs—similar to how a monorail train is a type of train, but a monorail is not a train, but the thing the monorail train runs on. Some people use the word “subway” to mean subway train, though. There are many types of trains.

While both are rail-based transportation, the primary difference between a "train" and a "subway" (or "metro") lies in their purpose, power source, and geography. A subway is a rapid transit system designed for high-capacity, frequent travel within a specific urban area, typically powered by a "third rail" or overhead lines and often separated from other traffic via tunnels or elevated tracks. In contrast, "train" usually refers to heavy rail or commuter rail that connects different cities or suburbs over longer distances. Regional trains often have their own engines (diesel or electric locomotives) and operate on a less frequent schedule with more comfortable seating and amenities like restrooms or food service. Terminologies also vary by region; in London, it's the "Underground," in Paris, the "Métro," and in New York, the "Subway." Essentially, subways move you through the city, while trains move you between cities or into the city from the outskirts.