Because high horsepower is required for high-speed operation, the diesel is, therefore, less desirable than the electric for high-speed passenger services and very fast freight operations.

Diesel trains are often perceived as "slow" compared to electric trains primarily because of weight, gear ratios, and torque management. A diesel-electric locomotive is essentially a "rolling power plant"—it carries a massive internal combustion engine, a huge fuel tank, and a heavy electrical generator, all of which add incredible weight to the train. This high "mass" makes it much slower to accelerate from a standstill compared to electric trains, which draw power from overhead wires and have a much higher power-to-weight ratio. Furthermore, diesel engines have a very narrow efficient RPM range. To reach high speeds, they would require a massive, complex mechanical transmission (with dozens of gears) which would be too heavy and prone to failure. Instead, they use electric traction motors to turn the wheels. While this is efficient for hauling heavy freight at 60–70 mph, the power loss during the conversion from diesel to electricity makes it difficult to maintain the ultra-high speeds (180+ mph) achieved by pure electric bullet trains. In 2026, while diesel remains the workhorse for heavy hauling, its inherent mechanical complexity and weight keep it firmly in the "medium speed" category of rail travel.