The SIFA alarm is a dead man's switch that the engineer must press periodically to ensure that the driver is awake. If the switch is not pressed the train applies the brakes.
In the BR442 the SIFA alarm switch is located at the engineer's feet. To get this achievement look down at your feet while driving, locate the switch and mash the A button. You don't have to wait until you hear an alarm. I did this between stops and completed it in under one run. Although I wasn't counting, it seemed like the achievement popped long before I got to 500.
Edit: Dynamite Andy pointed out that you can just mash the B button. There's no need to look at the switch as I suggested.