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Both are located to the side of the train, as the space between the running rails is occupied by an aluminum plate, as part of stator of the linear induction propulsion system used on the Innovia ART system. Power conversion for a DC system takes place mainly in a railway substation where large, heavy, and more efficient hardware can be used as compared to an AC system where conversion takes place aboard the locomotive where space is limited and losses are significantly higher. The low-frequency AC system may be powered by separate generation and distribution network or a network of converter substations, adding the expense, also low-frequency transformers, used both at the substations and on the rolling stock, are particularly bulky and heavy. Most electrification systems use overhead wires, but third rail is an option up to 1,500 V. Third rail systems almost exclusively use DC distribution. As alternating current is used with high voltages, this method of electrification is only used on overhead lines, never on third rails for safety reasons. More recently, the development of very high power semiconductors has caused the classic DC motor to be largely replaced with the three-phase induction motor fed by a variable frequency drive, a special inverter that varies both frequency and voltage to control motor speed.

Third rail is more compact than overhead wires and can be used in smaller-diameter tunnels, an important factor for subway systems. Both the transmission and conversion of electric energy involve losses: ohmic losses in wires and power electronics, magnetic field losses in transformers and smoothing reactors (inductors). However, the higher voltages used in many AC electrification systems reduce transmission losses over longer distances, allowing for fewer substations or more powerful locomotives to be used. 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. The standard-frequency AC system may introduce imbalance to the supply grid, requiring careful planning and design (as at each substation power is drawn from two out of three phases). Although the supply has an artificially created earth point, this connection is derived by using resistors which ensures that stray earth currents are kept to manageable levels. This created tension between generals and their operators. The FCC required operators to black out programming that comes in from distant markets and duplicates a local station’s own programming (if the local station demanded it).

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. Factors that affect resistance and thus loss include temperature, spiraling, and the skin effect. The higher the voltage, the lower the current for the same power (because power is current multiplied by voltage), what is electric cable and power loss is proportional to the current squared. The majority of modern electrification systems take AC energy from a power grid that is delivered to a locomotive, and within the locomotive, transformed and rectified to a lower DC voltage in preparation for use by traction motors. 420 V DC, and a top-contact fourth rail is located centrally between the running rails at −210 V DC, which combine to provide a traction voltage of 630 V DC. The additional rail carries the electrical return that, on third-rail and overhead networks, is provided by the running rails.

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. 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. Electric trains need not carry the weight of prime movers, transmission and fuel. The transmission network is usually administered on a regional basis by an entity such as a regional transmission organization or transmission system operator. A few lines of the Paris Métro in France operate on a four-rail power system. The London Underground in England is one of few networks that uses a four-rail system. Apart from only requiring a simple control system for the motors, the smaller size of urban operations meant that trains were usually lighter and needed less power. On the other hand, the higher voltage requires larger isolation gaps, requiring some elements of infrastructure to be larger.