On the main line these locomotives take the current from the trolley wire by means of the trolley pole, but when the place for gathering is reached, the connection is made by means of the flexible cable, and the trolley pole fastened down so as not to be in the way. This allows the locomotive to push the little cars into the rooms far removed from the main line, with passages too low and narrow to allow the use of the trolley pole. By the time the last cars have been delivered the first cars of the train have been filled, and the process of gathering may be begun at once, and the loaded train made up for the return trip. With such a locomotive two men can distribute and gather up from one hundred to one hundred and twenty cars in an ordinary eight-hour working-day, hauling from three hundred to three hundred and fifty tons of coal. In certain regions, a system of third-rail current-supply is used, this rail being also a tooth rail with which a cog on the locomotive works frictionally. For climbing steep grades this system of cogged rails has many advantages over other systems. Another type of electric locomotive used in some mines is a self-propelling or automobile one equipped with storage batteries. Such locomotives do away with the inconvenience and dangers of contact rails or trolley wires, but are heavy and expensive. A compromise locomotive, particularly useful for gathering, is one equipped with both trolley pole and storage batteries. This locomotive is so made that the storage batteries are charged while it is running with the trolley connection, so that no time is lost in the charging process. Such locomotives have been found very satisfactory for many purposes, and but for the imperfections common to all storage batteries would be ideal in many ways. They can be worked over any improvised track, regardless of distance, which is an advantage over the flexible-cable system where distances are limited by the length of cable; and the first cost of the battery is no more than the outlay on trolley wires and supports. It is also claimed that the cost of maintenance is relatively low, but it is doubtful if it equals the trolley or third-rail systems in this respect. Closely allied to the systems of traction by electric locomotives, is the modern electric telpherage system. Until quite recently the haulage of ores and other raw materials used in mining, when done aerially, has been by means of travelling rope or cable. When distances to be travelled in this manner are short, such as across streams or valleys, where no supports are used, the term "cableway" is generally applied; but where the distance is so long that supports are necessary, the term "tramway cable" is used. It is to these longer systems that electric telpherage is particularly applicable.