Many of the problems of extracting the Sahara's resources lie in the difficulty of the terrain itself and the expense of building and maintaining adequate transport routes. No railway penetrates more than a short distance into the desert, and only a few roads make the crossing from north to south. Trucks and buses are being used increasingly for long-distance hauling, although the traditional camel caravans are still used. Many goods and passengers are transported by air, and a number of the larger oases have scheduled air service.