In the airforce 400 Hz is used, In general the higher the frequency the more compact the motors and transformers hence less weight.
Commercial aircraft too. That incessant high-pitched whine that continues even after they shut off the engines is 400Hz transformer hum.
25Hz was not used only in rail service. Early industrial systems used 25 Hz because many applications called for a slower RPM motor. There is a fascinating story about how 25Hz came to be a standard in the US.
Early Westinghouse (pre-Tesla) equipment was 133Hz single-phase. Tesla's designs were all 60Hz and so superior that Westinghouse abandoned previous designs. Industrial power called for low frequencies due to the demand for low motor speeds, but these frequencies were not suitable for incandescent lighting because they caused lamps to flicker.
Tesla proposed 60Hz for lighting and 30Hz for power, but during the design stage of the Niagara Falls power project, there was a lack of communication between the mechanical and electrical engineers. It seems the turbines had already been ordered, with a design RPM not suitable for 30Hz. The power system was changed to 25Hz, and since Niagara was such a large project, 25Hz became the standard.
It would really make much more sense to have 30&60, or 25&50, since two alternators with different numbers of poles could then be driven by the same shaft.
25Hz power systems become more scarce by the year.
