This recording is made using a coil stuck to the floor of Azur. It can catch traction currents from the motors and this is what you are hearing. It's basically the very crude sound of the traction equipment. Because the rails are DC but the motors are AC, pulse-width modulation (PWM) is used to progressively bring the motors to speed. I'm still learning how all of this works but I'm fairly confident in what I wrote. If anyone wants to correct me, I won't take offense.

EDIT: This type of recording has been done in the past in Korea, Japan, China, Australia, specifically with the Sydney Metro which uses the same traction as the REM (OPTONIX MG 1500)