In 1977, a tunnel-cleaning train was completed. This consisted of two 1938 driving motor cars, and three purpose-built vehicles. The middle vehicle of the five contains a fan unit, which supplies large volumes of low pressure air to a series of nozzles, which disturb the dust on the tunnel walls and track. The cars on either side of it draw the dust-laden air into filter chambers, and are fitted with conveyors for discharging the dust at depots. To enable the train to operate sufficiently slowly for the cleaning process to be effective, a hydraulic drive was fitted to one of the motor cars, with settings to allow speeds of 0.5 mph (0.8 km/h), 1.5 mph (2.4 km/h), 4.5 mph (7.2 km/h) and 6 mph (9.7 km/h). Up to 6 tons of dust can be held in the filter bags, and the units are fitted with carbon dioxide and water mist fire fighting equipment, because of the combustible nature of fine dust particles. There were teething problems when the train began work in 1978, but by 1980, these had been resolved.