This affects all Peninsular Corridor DLC if you own the Baby Bullet DLC (on PS4 at least), you do not need to be using the MP36 at the time. Whenever you are within any fairly close range of an MP36 when it is due to start a journey, the HEP start-up sound plays incredibly loudly (as if you were in the locomotive). “Fairly close” in this instance is anywhere where you might be able to see an MP36, which could be a hundred yards away or more. The sound just plays at full blast. I’ve had this make me jump when playing with the MP15 in the yard and while trainspotting (about a hundred yards from the nearest MP36 locomotive) outside SF station, it really is that loud. It’s spoiling my enjoyment of the route.