Overhead Bridge Cranes can be divided into two groups: Top-Running Bridge cranes and Under-Running Bridge cranes (sometimes called Under Hung). A Bridge crane system consists of three major components: the Bridge crane, which traverses across the runway, the hoist & trolley, which traverse across the bridge and lift up and down, and the runway, which is tied to the building structure. Bridge cranes are primarily used indoors and each is modified to fit inside a specific building. Once installed, they usually remain inside the building until they are no longer needed. The components of the Bridge crane (hoist & trolley) are relatively easy to remove, but having the runways tied to a building are time-consuming and rather expensive to remove from the building. Once removed from an existing structure, they must be modified in order to be installed in a new structure.