The distance between two sets of parallel railroad tracks is known as the "Center-to-Center" spacing (or track centers). In the United States and most of Europe, the standard minimum distance for modern main-line tracks is typically 13 to 15 feet (approx. 4 to 4.5 meters). This spacing is determined by the "loading gauge," which is the width of the train cars themselves (usually around 10 to 10.5 feet), plus a mandatory "clearance" to ensure that two passing trains do not collide or create dangerous aerodynamic pressure between them. For high-speed rail lines, such as those in Japan or the new European corridors, this spacing is often increased to 16 to 18 feet to account for the intense "air pressure wave" generated when two trains pass each other at 180+ mph. In older, legacy rail yards or narrow-gauge systems, the spacing can be as little as 11 feet, but modern safety standards increasingly demand wider gaps to allow for maintenance staff to stand safely between tracks. The distance is a fundamental engineering constant that ensures the safe and efficient movement of goods and passengers across national and international networks.