Train horns all hours of the night.

I recently visited San Diego and from our hotel all we could hear at night were incredibly loud train horns - and I'm talking 3am, 4am - it never stopped? Now I know why the hotel offered ear plugs. How can the city allow this? And the horns were not sounded one time, its at least 5 times within a 5 minute stretch. Would definitely put me off staying downtown again.