Tunnels don't get any rain, but they get plenty of car exhaust, which leaves grime behind. How do you wash a tunnel? With a truck built especially of that purpose. The Swiss road construction company Colas Suisse has those trucks. They resemble a car wash turned inside-out, with plenty of arms ending in whirling brushes. See one of these machines in action.
The tunnel-cleaning machine uses no soap, because that would be bad for the environment. It gets the tunnel walls squeaky-clean purely with water and whirling brushes. -via The Kid Should See This