A train maintenance depot is the locations in which a fleet of trains are serviced and maintained. A maintenance depot will consist of a storage yard, a cab cleaning area, an inspection and light maintenance shed and a heavy maintenance shop. However, trains are always connected to an overhead line that has a constant current of electricity turning through it. Therefore, power to the overhead line must be isolated before maintenance can be performed on the train. This process is interlocked to ensure that access to the maintenance shed can only be granted once power to the train has been isolated.