Dsd is the driver safety device when driving a train you must keep your foot down on a pedal and after a certain amount of time a alert is sounded and you have to lift your foot up of the pedal and back down to reset the alarm. In game when you here a bleeping noise you just have to click B to reset it. Just a heads up all safety devices are turned of by default so you have to activate them via controls in the cab. Hope this is helpful if not just Google it
Knightfire1964 Also u only have between 2-3 seconds to acknowledge the alert or the emergency brakes will apply