Utility frequency, line frequency or mains frequency is the frequency level of the AC electricity used in transmission and distribution from electricity generation sources to customers. The world's two major utility frequencies are 50 Hz (most, but not all, of the Eastern hemisphere) and 60 Hz (most, but not all, of the Western hemisphere).
Many electric railways in Europe use a frequency of 16.7 Hz (one third of 50 Hz). Early electric railways sometimes used 25 Hz (half of 50 Hz). New electric railways usually use the standard mains frequency of 50 Hz. Some lines in the northeast United States still use 25 Hz.