Why are there two braids on the top of the tram?

■ Why are there two "pigtails" on the top of the tram? Tram is a common public transport, and its most striking place is two braided collecting poles on the roof. Trams are powered by electricity, while other buses are powered by gasoline. However, the electric energy used by the tram is not generated by itself, but the "braid" on the roof is placed on the overhead line to obtain energy. Therefore, trams must travel along roads with overhead wires, and cannot travel at will like other cars.