An elevated railway
A double-track line on riveted steel bents and lattice girders above the street, as on Chicago’s Union Loop (1897); or the same line in a walled cutting, as on Vienna’s Stadtbahn.
The rules it illustrates
One system of transit for the centre. Treat the city’s railways, elevated lines and streetcars as a single system serving the business centre, and plan their stations and routes together, relieving the elevated loop with subways and through routes.