Tokyo has multiple elevated railways that cross through the city. Under-railway spaces facilitate transversal movement and vehicular traffic, but also often create a negative impression to passersby. This study selects publicly accessible under-railway space and creates a taxonomy by focusing on their urban permeability, meaning the degree to which an urban area is permeated by publicly accessible space.
We conducted the research according to the following procedures. First, we selected the under-railway blocks that have under-railway spaces with commercial use or public use, such as a park or a kindergarten. After that we made units which were continuous among them. Also, we divided them when they were crossed by streets 14 m in width, which is the general width of the main road. Then we selected units with a length of 100 m or more and defined them as under-railway units. Then, we classify the under-railway blocks. For doing this, we consider their length, adjacency to the street, and presence of pedestrian alleys to check the relationship between under-railway blocks and the streets. In order to examine urban permeability, we examined the interface of the under-railway establishments with the street, we calculated the number of establishments along under-railway block (density) and number of access along under-railway block (accessibility). Next, we make a further classification of the under-railway blocks by looking at their position in relation to the station, the area, the programs of the establishments, and period of construction. As a result, we obtained a taxonomy, from which we can see that the types built before the Grade Separation Project of 1961, such as Ueno or Shimbashi stations, show a high permeability that result from the many small establishments. These establishments were built in the under-railway spaces after the elevated railway was constructed.
Also, we found under railway blocks built by the Grade Separation Project, located mostly in suburban areas, such as Nerima or Hachiman-yama stations, the under railway space is occupied by large establishments along the station with close interface. These types show lower permeability. The establishments were planned and built together with the elevated railway. However, among those built after the Grade Separation Project, we can also find types of higher permeability such as Ayase, Koenji or Nishi-Kasai Stations. These types connect with the surrounding area by arranging the establishments in high density and by subdividing the under-railway blocks by transverse alleys. In this way, the under-railway space, which has been negatively evaluated as a residual space, was utilized by collecting small establishments, opening their interfaces to the public, and subdividing the under-railway blocks by alleys. This research shows some examples how urban permeability can be implemented in under-railway spaces in order to make them an active part of the city.