The transportation network is often referred to as the "bones" of a city. It is easy to understand why; the streets form the underlying structure of a city, much like the bones form the underlying structure of the human body. The term has another side to it though; referring to the permanence of the network, and the difficulty in changing those bones of the city. When a route is laid on the earth, it is likely to remain through time and development. You can see this process occurring throughout the world; from the many roads and city plans of the ancient Roman empire still in use today to the famous Broadway that cuts through Manhattan's grid, originally a Native American trade path through the island.
One such bone that hasn't quite healed can be found cutting curiously across the very strict grid of the Mission district in San Francisco. One look at a satellite image of the neighborhood will reveal the bone's history in many forms; buildings cut at odd corners, diagonal parklets found mid-block, and oddly shaped parking lots.
The origin of this bone is the San Francisco-San Jose railroad, which operated from 1864 until the 1940's. Despite not being in use for over 70 years, this diagonal line is easily apparent when looking at a satellite image of the neighborhood. The route was originally designed to follow the flattest contours of the terrain, an important consideration in hilly San Francisco, even in the relatively flat Mission district.
The permanence of the SF-SJ train route is a strong reminder that what we build will be on this Earth for a long time. It will have a domino effect on future development that we cannot predict.
Today you can see the land of the former train route reincarnated in many forms, some pleasant and some eyesores to the neighborhood. Juri Commons is a beautiful, little park that runs through an entire block of former railroad land; however you are just as likely to end up with oddly shaped parking lots and unused land.
Below are several images of the bones of the former railroad as they appear in their current reincarnation: