DC PWM sound on a 319 Thameslink train

DC Pulse width modulation is an old 80s technology which was the forerunner of modern AC VF systems. The trains motor is turned off and on 100 times a second. The percent of each time period it is on increases as the train speeds up to reduce inrush current when it first starts. The frequency of the switching does not change, only the amount of the pulse that is high (true) changes. This gives a constant low hum as the train accelerates until it reaches the point (about 15 mph) when the motor is fed directly. http://beno.org.uk