50 vs 60
The optimal choice is a tradeoff for costs, losses and expected revenue opportunities. The biggest consumers tend to be industrial smelting plants with massive arc currents and special transformers required, followed by industrial motors.
It depends on the cost of adds and losses such as added copper needed or thinner silicate-coated transformer-steel core materials losses due to skin effects in conductors and eddy currents losses in magnetics.
Also 60 Hz motors could yield higher RPM in induction motors 3600 vs 3000 RPM with no load.
Mutual coupling is lower at 50Hz for the same materials, light flicker is greater
Airplanes use a higher frequency usually 400Hz in order to reduce core weight to achieve high efficiency.
AC vs DC
DC transformation was more expensive in large scale and with high voltage DC can be more problematic and needs more protection with latching effects and tribolectric charging effects.
HVDC is best suited for long haul distribution, where the Chinese are experts with 1GV technology installed around the world.
Why not change 50 or 60 Hz ? Cost of inertia vs savings is too great.
Other power sources
Diesel trains use DC batteries and generators to drive DC traction motors. THey do generate a lot of EMI however.
The Russians & others have experimented with MagnetoHydroDynamic (MHD) generators with no moving parts and fields > 1 T.
In some cases these generators run in the range of 4-6 kHz.
The Chinese now have built a Fusion Reactor 100 times hotter than the inside of the Sun based on Russian technology.
If they knew how to distribute HVDC in Edison's day, Tesla would have lost the battle which he won based on improved efficiency of transformer step-up and step-down of distribution costs, reliability of induction motors over brush-type DC and other reasons.