Best case: "I don't seem to be making any progress going up."
Worst case: "This is moving down way too fast! Aaaahhhhh!!!" -screamed by hundreds of people, many of which will die as they pile up at the bottom of the escalator.
Now imagine the brakes failing on an escalator that leads down into a subway with an electrified rail.
The weight of your body will set the escalator in motion and it will keep accelerating. As you approach the bottom end at ever increasing speeds you will have a few moments to contemplate on what a mistake it was to not take physics class in high school. Once you reach the end of the line your journey will end in a rather unpleasant manner due to the high velocity. You could get injured very seriously or even killed.
