US metro systems in cities such as Washington DC, Chicago, San Francisco, and Los Angeles share an interesting design feature. All of these systems have major segments where different routes share the same track. For example, Washington DC’s metro has three individual lines that share tracks from Rosslyn station to Stadium-Armory station. Meanwhile, Chicago’s rail system can have up to five different lines (including one special rush-hour service) share the same tracks along the city’s iconic elevated “loop”. This phenomenon where multiple train routes share the same tracks is called interlining and can be found in metros all around the world. However, most metro systems in other countries only interline some of their routes. US metros, on the other hand, interline nearly all of their routes. Furthermore, these interlined routes take center-stage in these metros, passing through their respective urban centers before fanning out into the surrounding suburbs. The main advantage of this design is that it allows commuters to directly travel in and out of a city’s downtown area without having to transfer between trains, something known as a one-seat ride . For example, in San Francisco’s BART, every single station (with the exception of its eBART light