The left and right lines of the subway in the United States are divided based on the direction of travel and the color coding system. Typically, the left line represents trains traveling in one direction, while the right line represents trains traveling in the opposite direction. This system is designed to streamline navigation and ensure that passengers can easily understand the direction of each train.

The color coding system is a key factor in this division. For example, the New York City Subway uses a color scheme where each line is assigned a specific color. Trains traveling in one direction are marked with the color associated with that line, while trains traveling in the opposite direction are marked with the color's opposite. This visual distinction helps passengers quickly identify which train to board.

Data from the New York City Subway, which is one of the largest and most complex in the United States, shows that this system is effective. According to the Metropolitan Transportation Authority (MTA), the color coding and directional division have significantly reduced confusion among passengers. The MTA reports that the system has helped to improve overall efficiency and safety, as passengers are less likely to board the wrong train.

The use of color coding and directional division is also seen in other major subway systems across the United States, such as the Washington D.C. Metro and the Chicago 'L'. These systems have adopted similar strategies to ensure clear and effective navigation for their passengers. The consistency of this approach across different cities highlights its effectiveness and widespread acceptance in the United States.