That's what the monorail says. I'm taking Spanish, so I was interested in hearing it when we got there last May and it does say what you wrote, "Por favor, manténgase alejado de las puertas." I think a more literal translation would be something like, "Please keep yourself away from the doors." I think "stand clear" is an English idiom and so the Spanish doesn't translate it word for word, but instead says it in a way that conveys the same meaning if not the same words.
-Dorothy (LadyZolt)