Likely due to some exponential thing with the switching frequency following the soft acceleration, as the speed as far as I know increases according to a cubic

Likely due to some exponential thing with the switching frequency following the soft acceleration, as the speed as far as I know increases according to a cubic function until about half of full acceleration is reached, and from there on it goes over to quadratic until the train hits about 80 km/h, as they lack power at around that speed to keep the acceleration at the usual 0.9-1.1 m/s^2. And the harmonics are exponential, with log_2(#of_notes/12) giving you the relative frequency difference for a given number of notes spacing.