I've noticed that there is sometimes, for a relatively short length, a set of narrow rails inside the rails that the train rides on. It seems these inner rails usually are present on bridges. I am guessing they are intended to help guide a derailed train in places where it would be catastrophic if the train strayed any consequential distance from the main rails (like on a bridge over water)? For years I've wondered what these extra rails are for. An explanation of their real purpose would be appreciated. As with everything else, I am sure they have some unique name.