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Locomotive haulage is wide-spread in tunnel driving as it is versatile, leaves the way clear and safe when not being used, and packs a lot of power into a small place. Battery power dominates in most drivages as it is convenient, economical, compact and non-polluting. However, in long drives or those requiring a high spoil-removal rate, diesel engines specially designed or adapted to underground conditions, are used where the power drain on batteries would make them impractical. Trolley electric locos would be more economical but the provision of the overhead power supply usually makes them unsuitable for tunnel driving unless the end-use of the tunnel can be incorporated such as in rail transport tunnels or main mine haulages.