wrote

Diesel-electric locomotives run on diesel fuel, not electricity.

A big conventional diesel engine drives a conventional generator which
in turn powers electric motors.

Why don't they just drive the thing directly from the diesel engine?

The electric drive eliminates the need for a huge transmission.

Plus another very important factor.

The electric motors turn into big generators, when the train starts pushing
the locomotive, like when coming down a fairly steep grade.

Rather than overheat or wear out the car brakes, the wheel motors generate
massive amounts of electricity that the engineer directs into a huge banks
of resistors, which act like giant electric heaters, heating the outside
air. Only when all of the resistors are running at full power, does the
engineer begin selecting the car brakes.
--
Jim in NC