Lisboa is built on hills. Mule or horse trams could only go up the steep gradients with the use of extra animals. As the animals were the largest cost of the whole tram operations, it was a too costly affair to have many or long steep inclines within the network. In 1873 in San Francisco a system was introduced employing a cable running continuously in a conduit below the track, which could be picked up or released by a gripper with metal jaws suspended from the car and passing through a slot in the road. Many such systems were built in America and other countries. Also in Lisboa three cable tramlines were constructed, all with a gauge of 900 mm.