In the former Soviet Union, <answer>electric</answer> traction eventually became somewhat more energy-efficient than diesel. Partly due to inefficient generation of <answer>electric</answer>ity in the USSR (only 20.8% thermal efficiency in 1950 vs. 36.2% in 1975), in 1950 diesel traction was about twice as energy efficient as <answer>electric</answer> traction (in terms of net tonne-km of freight per kg of fuel). But as efficiency of <answer>electric</answer>ity generation (and thus of <answer>electric</answer> traction) improved, by about 1965 <answer>electric</answer> railways became more efficient than diesel. After the mid 1970s <answer>electric</answer>s used about 25% less fuel per ton-km. However diesels were mainly used on single track lines with a fair amount of traffic  so that the lower fuel consumption of <answer>electric</answer>s may be in part due to better operating conditions on electrified lines (such as double tracking) rather than inherent energy efficiency. Nevertheless, the cost of diesel fuel was about 1.5 times more (per unit of heat energy content) than that of the fuel used in <answer>electric</answer> power plants (that generated <answer>electric</answer>ity), thus making <answer>electric</answer> railways even more energy-cost effective.
What type of trains became more energy-efficient in the former Soviet Union?
electric