Is there an optimum internal diameter for subway tunnels in terms of economic and technical viewpoints?

Subways, one of the most popular modes of the rapid transit systems, play an important role in maintaining the sustainability of transportation in large metropolitan areas. This study focuses on the optimum dimension of the internal diameter of subway tunnels that creates a space for train passage that is large enough to preventing collisions with electromechanical and wayside facilities and emergency walkways and is small enough to minimize costs. The discussion hinges around how a train in the worst-case situation of a super-elevated track with a minimum curve radius can pass through a tunnel equipped with wayside facilities without any accidents while satisfying all the economic aspects of the project. Limiting the speed of a train in response to the confined area between a train’s dynamic envelope and emergency walkways leads to extended travel times, excess wear on the wheel and rail and even changes in the amount of electricity consumed by the train. All the mentioned factors result in an operational cost and the total cost is achieved by summing up this cost and the construction costs for different tunnel diameters. An economic analysis is performed by comparing the costs of construction and operation for different tunnels against the benefits of passenger revenues. Generating net-present-value diagrams for tunnels with different internal diameters as a function of time shows that a double-tracked single-tube tunnel with an 8.40-m internal diameter is the optimum from both an economic point of view and being able to match technical specifications.