Power-only rails can be mounted on strongly insulating ceramic chairs to minimise current leak, but this is not possible for running rails, which have to be seated on stronger metal chairs to carry the weight of trains. DC voltages between 600 V and 750 V are used by most tramways and trolleybus networks, as well as some metro systems as the traction motors accept this voltage without the weight of an on-board transformer. In 1970 the Ural Electromechanical Institute of Railway Engineers carried out calculations for railway electrification at 12 kV DC, showing that the equivalent loss levels for a 25 kV AC system could be achieved with DC voltage between 11 and 16 kV. Between 2012 and 2016, the electrification was converted to 25 kV 50 Hz, which is the countrywide system. It was formerly used by the Milwaukee Road from Harlowton, Montana, to Seattle, across the Continental Divide and including extensive branch and loop lines in Montana, and by the Delaware, Lackawanna and Western Railroad (now New Jersey Transit, converted to 25 kV AC) in the United States, and the Kolkata suburban railway (Bardhaman Main Line) in India, before it was converted to 25 kV 50 Hz.

Parts of the original electrified network still operate at 25 Hz, with voltage boosted to 12 kV, while others were converted to 12.5 or 25 kV 60 Hz. Approximately, the power flowing over an AC line is proportional to the cosine of the phase angle of the voltage and current at the ends. The lower current reduces line loss, thus allowing higher power to be delivered. The first transmission of single-phase alternating current using high voltage came in Oregon in 1890 when power was delivered from a hydroelectric plant at Willamette Falls to the city of Portland 14 miles (23 km) down river. In the 1960s the Soviets experimented with boosting the overhead voltage from 3 to 6 kV. Third rail is more compact than overhead wires and can be used in smaller-diameter tunnels, an important factor for subway systems. An early advantage of AC is that the power-wasting resistors used in DC locomotives for speed control were not needed in an AC locomotive: multiple taps on the transformer can supply a range of voltages.

These drives can run equally well on DC or AC of any frequency, and many modern electric locomotives are designed to handle different supply voltages and frequencies to simplify cross-border operation. This can cause electrolytic damage and even arcing if the tunnel segments are not electrically bonded together. However, elastomeric rubber pads placed between the rails and chairs can now solve part of the problem by insulating the running rails from the current return should there be a leakage through the running rails. AC power is used at 60 Hz in North America (excluding the aforementioned 25 Hz network), western Japan, South Korea and Taiwan; and at 50 Hz in a number of European countries, India, Saudi Arabia, eastern Japan, countries that used to be part of the Soviet Union, on high-speed lines in much of Western Europe (including countries that still run conventional railways under DC but not in countries using 16.7 Hz, see above).

Several countries have announced plans to electrify all or most of their railway network, including Indian Railways and Israel Railways. P. M. Kalla-Bishop, Future Railways and Guided Transport, IPC Transport Press Ltd. 2024: Interactive TV experiences and integration with smart home devices became more prevalent, transforming how viewers interact with their TVs. The rise of smart TVs, on-demand content, and interactive viewing experiences continues to challenge traditional broadcast models. Electrical cable jacket material is usually constructed of flexible plastic which will burn. Some of these, particularly Victorian mains that predated London's underground railways, were not constructed to carry currents and had no adequate electrical bonding between pipe segments. The London Underground in England is one of few networks that uses a four-rail system. The same system was used for Milan's earliest underground line, Milan Metro's line 1, whose more recent lines use an overhead catenary or a third rail. 6-7 UIC conducted a case study for the conversion of the Bordeaux-Hendaye railway line (France), currently electrified at 1.5 kV DC, to 9 kV DC and found that the conversion would allow to use less bulky overhead wires (saving €20 million per 100 route-km) and lower the losses (saving 2 GWh per year per 100 route-km; equalling about €150,000 p.a.).

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