Another key is getting the cost of tunnelling down. While MM1 is costing about $1b per kilometer, Perth is currently building an underground railway for $200 m per kilometer, at much lower cost because it is being built beneath low-density suburbs and the airport. The 3 stations on this line are simple box structures built without needing to maintain operating roadways above, which means construction cranes can be used to lower costs. One station will be beneath a roadway, but the road is diverted during construction. Melbourne Central was built this way, with Latrobe St diverted south into where the Melbourne Central shopping centre has since been built.