Crossrail will use eight tunnel boring machines (TBMs) to construct the new tunnels under London. These huge machines will work 24 hours a day beneath the streets of London, excavating large volumes of ground and erecting the concrete tunnel lining. Each machine has a rotating cutter head at the front, and a series of trailers behind, housing all the mechanical and electrical equipment required for the excavation of material. To construct the 42km of tunnel required for Crossrail the tunnel boring machines will undertake ten individual tunnel drives to construct the 6.2m diameter rail tunnels. Each TBM weighs approximately 1000 tonnes and will be up to 140m in length with an external diameter of 7.1m. This allows for an inside tunnel diameter of 6.2m once the concrete tunnel segments are in place. Each TBM will be operated by a “tunnel gang” comprising of around twenty people – twelve people on the TBM itself and eight people working from the rear of the machine to above ground.