Have you noticed burning smells when a C151 train brakes? One theory is because, with the new CBTC system, the system uses the mechanical brakes much earlier than that of a human usually will. This results in more work done by the brakes, as the speed when the mechanical brake engages is higher, which causes the brakes to emit the burning smell. This is also evident in some newer models, but more prominent in the older C151.
So, how does trains usually brake before starting to use the mechanical brakes? Apparently, the traction system is used to decelerate the train before the mechanical brakes take over at lower speeds. Hence, this also explains why you can hear the traction system, and then the normal braking sound, at lower speeds, when the handling over is done. Emergency braking also uses the mechanical brakes, hence the burning smell can also be detected when the train brakes hard.
Hope the above explains some things! Feel free to add on below.