AC transmission has inductive resistance that DC transmission does not have, as well as DC using the entire wire instead of just the area near the surface.

Getting this somewhat back the topic of railroads...

In the early 20th century, both AC and DC electrification was used, with AC having the advantage of easier long distance transmission of power, and DC having the advantage that a train going downhill could use regenerative braking to pump current back into the wire from its traction motors (the same idea as dynamic braking on a diesel), so a downhill train could help power an uphill train.

Because of problems with keeping the power produced by the locomotive in phase with the line current, this could not be done with AC electrification. When I learned about running motor-generator sets in the Air Force, we were taught to watch the phase light. When it was on, the power being produced by the generator was out of phase with the power on the line, when it went out, the two were in phase and it was OK to hit the load contactor. Hit it when the power was out of phase, you can have an explosion. This is considered a Bad Thing.

Now, with the same technology that allows DC to be converted back to AC after long distance transmission, AC electric locomotives can pump power back into the wire when they are in regenerative braking, with the electronics keeping the inverter in phase with the wire. A loadedgrain train coming down an electrified Feather River Canyon could help an eastbound up the hill, for instance.

I guess if a railroad ran ore or coal from a high location to a low location and brought empties back, it could, in theory, produce more power than it consumed.