air and evacuation of exhaust air are connected to a distribution duct for supply air and a distribution duct for exhaust air via each an externally insulated duct having adjustable damper for supply air and negative pressure and flexibles compensators for the compensation of length variations of the ducts the distribution duct for exhaust air 4 is connected to a hot air fan 10 to maintain the air flow and pressure in the duct network the number of revolutions of the hot air fan is controlled by a frequency converter to maintain a desired flow of hot air to the tunnel the air flow is measured by a flow sensor that regulates the rotational speed of booster fans the hot air generator 5 is installed after the hot air fan the hot air generator 5 contains a number of liquefied petroleum gas heated duct burners the hot air generator 5 is provided with its own control system and high pressure fan for the combustion air fuel flow for the combustion air is controlled by a set value from the control system and temperature sensors in the supply air flow in each end of the de icing plant there is a so called gate curtain having each an air curtain that blows air against the truck sides to prevent the ambient air from being sucked into the evacuation ducts in the end of the gate curtains there is a remote controlled hinged door which normally is closed when de icing is not carried out above the gates there is installed a flexible wall of electrically insulating material for the passage of aerial line and collector shoe of locomotive and drive trucks with the gates closed heating and.