The "wet steel" comment reminded me-- wet trolley tracks are a real danger around where I live. In my area, one has to cross trolley tracks at odd angles in order to go around parked cars, make turns, and sometimes in order to cross a spot where the trolley route takes a turn. Even though you know the danger is coming, it's difficult to manage.
One thing that scares me in the "traction surprise" category is pavement that looks clean enough but has a lot of slick dust or extremely small rock/debris on it.