The railways in Switzerland, Germany, Austria use 15,000 Volts single phase at 16 2/3 Hz frequency.
Earlier series traction motors designed for dc could also run on ac with reduced sparking on lower frequencies.
In the USA was / is? 25 Hz in use for electric railway traction.
Lower frequencies means bigger machines.
The main reason the airforce opted for 400 Hz is to reduce the size and weight of electrical equipment.
Look at a switch mode power supply. It runs on many kHz and are very compact in relation to their power output.
