Choice of Current and Voltage for North American Commuter Rail Electrifications

Engineers and planners today face challenges of assessing and adopting emerging propulsion technologies for commuter rail, particularly battery and hydrogen power. Early 20th century experiences with electrification technology may offer useful perspectives. When electrifying in the early 20th century, the most important decision facing North American commuter railroads was choosing electrical specifications—first between direct current (DC) and alternating current (AC), and then selecting a voltage. As a result, today’s electrified railroads present a sometimes-confusing array of specifications. Beneath the seeming disorder of specifications for legacy installations lies a logic based on the timing and circumstances of electrification, and on the ultimate size and purpose that railroads envisioned for these electrifications. Only low-voltage DC, delivered via third rail, was available for the earliest electrifications. By 1907, AC had entered the scene, and various overhead-wire DC specifications appeared between 1911 and 1930. Since the mid 20th century, 25,000 V AC at commercial frequency has become the default standard for new electrifications (and re-electrifications of older installations). Yet, even this standard has not been adopted for all reinforcements of overhead-wire electrifications, because of the historical overhang of earlier decisions. In situations where standardization of electrical specifications is not feasible, interoperability of equipment may be a more important priority.