What causes the whining sound when a BART train starts up?

Air brakes is a possible. If it is a diesel it could be a turbocharger. If it is electric it may be tight bearings on the motors. But if it is steam it may be a pressure valve releasing after it gets up to pressure. Here in Victoria (Australia) we have a "C Cl" diesel locomotive that gives off a heavy diesel hum with a turbocharger whistling behind the hum. It sort of sounds like a jet engine revving up. WOW awesome!!!