(no return on investment). As traffic density increases, the ratio of diesel to electric yearly expenses (including depreciation) increases. In an extreme case (traffic density 60 million tonne-km/km, and 1.1% ruling grade), diesel operating costs (including depreciation) are 75% higher than electric. Thus it really pays to electrify lines with high density traffic. Electrical systems Voltage and Current The USSR originally started at 1500 V DC (later converted to 3000 V in the 1960s) in the early 1930s selected 3,000 V DC for mainline electrification. Even then, it was realized that this 3kV voltage was too low for the catenary but too high to be optimal for traction motors. The solution to the problem was to use 25 kV AC for the catenary and provide on-board transformers to step down the 25 kV to a much lower voltage, after which it was rectified to provide a lower voltage DC. But it wasn't until the late 1950s that AC electrification became significant. Another proposal was to use 6kV dc\ and reduce the high voltage DC with power electronics before it was applied to the traction motors. Only one experimental train set using 6 kV was made and it only operated in the 1970s but was discontinued due to the low quality of its electrical equipment. In the final years of the Soviet Union, a debate was in progress as to whether the 3,000 V DC system should be converted to the standard 25 kV system or to a 12 kV DC system. 12 kV DC was claimed to have the same technical and economic advantages as 25 kV AC, while costing less and putting a balanced load on the nation's AC power grid (there is no reactive power problem to deal with). Opponents pointed out that such a move would create a third standard electrification system in the USSR. One proposal using 12 kV was to create a new locomotive that could operate under both 3kV and 12kV wires. It would convert 12kV to 3KV using power electronics and then use the 3kV (obtained directly if under a 3 kV wire) to power induction motors also using power electronics to drive them. Examples of electric locomotives 3 kV DC 2ES10 ChS2 ChS7 VL10 VL11 25 kV AC ChS4 ChS8 EP200 VL60 VL80 VL85 E5k Dual voltage EP10 EP20 VL82M See also Elektrichka Electrification of Saint Petersburg Railway Division History of rail