Deep-level tube lines

Deep-level tube lines were built by tunnelling shields and are tunnels that are deep underground that allow trains to run through them. They are commonly found on the London Underground. An example is the Victoria line.[1] The depth creates ventilation problems. Powerful fans must push clean air very far underground. In the event of an emergency the people trapped on-board cannot get off due to the insufficient space inside the tunnel compared to the subsurface line.