Narrow Inner Rail Set

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.

You guessed their purpose. They are called "guard rails".

Les

Guard rails are most common on bridges, tunnels would be another place. Any place where a derailment would damage a lot more than just the rolling stock. Trolley and transit systems have guard rails in many tight places like curves. Next to switches in normal speed territory on conventional railroads are guard rails opposite the frog to prevent a move from splitting a switch. One truck going on one track while the other goes down the siding. In low speed yard territory the frogs are usually self guarded. The guard is an intergral part of the frog.