Diesel-electric locomotives are more suitable than purely electric ones on non-electrified lines.
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Decompose: 柴 (chái) here is short for 柴油 (cháiyóu, diesel - literally "firewood oil") + 电 (diàn, electric) + 机车 (jīchē, locomotive). So: "diesel + electric + locomotive" = a locomotive whose diesel engine drives an electric generator.
