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this post was submitted on 16 Mar 2025
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My GUESS, and I have no knowledge of this, but my guess is using oil to generate steam is less efficient than simply running a diesel engine.
Oil -> Steam -> Drives Engine
Diesel -> Drives Engine
I assume it has a lot to do with efficiency, and correct me if I am wrong, but I think it goes Diesel -> Generator -> Traction Motors
You are correct. It relates to the torque (rotational force) that can be produced at different RPM.
Diesel engines have a narrow range of RPM where the torque is highest. At low RPM, the torque of a diesel engine is low. That's why trucks have to shift into a low gear to start moving and shift multiple times to get up to speed. The driver uses the different gears to keep the engine running within the optimal range of RPM.
Electric motors have high torque at low RPM, so they can start a heavy load moving without having to have any complicated gearbox between the motors and drive wheels.
In a diesel-electric locomotive, the diesel generator can be designed to run at an efficient speed without being affected by the mass of the train. The electricity it generates powers the electric motors which have the necessary torque to handle moving the train.
For anyone scrolling by and curious about this, this is caused by the combination of physical and electrical resistance. In a typical engine, RPM and torque go up together because it requires more force to get to those higher RPMs (IIRC this is called positive correlation). In a circuit, you have to kind of convince the electricity that it would be better off somewhere else (by connecting to a ground, this is due to electrical resistance), so you have to give it a heavy upfront load to get it going which causes a lot of torque due to the physical resistance