Caltrain runs a screeching loud horn and rumbling noise everywhere it runs. I stayed at a hotel recently (Sequoia Inn, 526 El Camino) - and I was forced to use ear plugs to get any sleep while the Caltrain was running. And I'm a heavy sleeper.
In my area, the trains only get active at around 11:30pm, an continue their racket through the entire night. I'm regularly awakened by needlessly long honking around 4am. It's awful.