This weekend, while in PDX, I was riding the light rail system and was reminded of a question that's been bugging me since I lived there 10+ years ago.....what are the extra rails for?
If you look at the picture, you'll notice a second set of rails running inside of the ones the train actually rides on.
They aren't for power, that comes from overhead. They primarily seem to exist on hills and curves, sometimes there's only one of them, sometimes they are even closer to the real tracks.
Based on the level of rust they show, compared to the active rails, they aren't touched by the trains in normal operation. My only guess is some sort of anti-derail system, but if that's it, I don't get how it would work.