Heavy lifting is done by machines such as overhead cranes or forklifts. Power is usually transmitted through overhead power lines. Containers handled by means of lifting gear, such as cranes, overhead conveyors, etc. A compromise between undergrounding and using overhead lines is installing air cables. The lines may be overhead or underground. These lines are diesel operated with no overhead wiring. Trolleybuses are electric buses that receive power from overhead wires for mobility. These are diesel only lines with no overhead wiring. This system filters out the voltage induced in the rails from the overhead lines. Luggage space in the overhead compartments is very limited. DC filters are required only for HVDC transmission systems involving overhead lines. The estimated minimal overhead for each is 300 words. This usually precludes operation of double-stacked wagons on lines with overhead electric wiring. This includes costs for material, parts, labor and overhead. This is the cost of Exec Shield's low overhead.