One of the Holy Grails of transport policy has thus been a way to turn these legacy lines into something like a modern metro system. The solution developed by many transit authorities is known as through running. This means taking suburban lines on opposite sides of the city, and joining them up by running trains through the city center, usually in a tunnel. This tunnel does not necessarily have to be very long: in some cities, only a handful of kilometers is needed, because the rest of the infrastructure (the legacy lines that are joined together by the tunnel) is already there. For the price of a comparatively small section of tunnel, a large number of suburbs can get metro-like service directly into the city center.
Through running has convergently evolved in many places. This article looks at two of them, Munich and London. Munich is distinctive in having used through running to create what is probably the best transit network on earth, relative to the size of the city. London is distinctive in the sheer size of its legacy network, due to its enormous size in the nineteenth century. This means that through running has offered it opportunities of unique scale and complexity.
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