Tunnels have been built from time immemorial for various purposes, such as defence, assault/ escape and normal traffic across fortifications and water bodies. In Indian history, references are available to tunnels connecting forts and points of escape and private passages for the royalty from the palaces to river fronts and temples. Transportation tunnels can be further classified as: railway tunnels, highway tunnels, pedestrian tunnels, navigation tunnels and subway tunnels. Railway and highway tunnels are similar in nature and normally refer to surface-to-surface route tunnels. Urban underground railways are mostly in the form of tunnels, otherwise known as tubes. Navigation tunnels are similar to highway tunnels. The conveyance tunnels cover those provided for water supply, sewerage and those provided for housing pipe lines for fluids or conveyor belts, as in case of mines, and power houses. The longest tunnel in USA is the replacement Cascade Tunnel in Washington State.