Although many of the major subway systems have significant portions of their track running on the surface, the main sections are run in tunnels. Often these tunnels are run along underneath the city streets – to avoid disturbing buildings – and constructed by the cut and cover technique. This consists simply of digging huge trenches down the middle of main roads which happen to run in the right direction, lining the floor and walls with brick or concrete to stabilize and strengthen them, and then putting girders from wall to wall over the top on which a new road foundation and surface can be laid. This can cause a great deal of road traffic disturbance while it is being done, but it is the cheapest method and often adopted for that reason. The rails and ancillary equipment are laid in the trench after the roof has been restored.

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