Railroads are efficient but diesel-electric engines are not an especially efficient way to move trains. Even with diesel engines, truck traffic generates nine times as much greenhouse gas emissions per ton mile as trains. But diesel-electric is not efficient due to carrying the diesel electric generator on its back.

Diesel-electric locomotives are significantly more efficient than the steam engines they replaced, but they are less efficient than modern electric locomotives. A 2026 efficiency analysis shows that diesel locomotives convert roughly 30–40% of their fuel's energy into movement, plagued by "combustion losses." In contrast, electric locomotives achieve 90%+ efficiency, as they don't carry the weight of an engine and can recover energy through regenerative braking (saving 12–33% of power). However, diesel remains a high-value choice for "long-haul" freight and remote lines because it requires no expensive overhead wire infrastructure. Compared to old steam locos, which were only 7–10% efficient and required constant maintenance, diesel is a premier workhorse; yet for high-density corridors in 2026, the "energy-efficiency crown" belongs firmly to the fully electric rail systems that power modern high-speed transit.