Rail question

What is the purpose of the having rails between the normal rails? I know they have "hybrid" tracks that have two different gauges but not really in the US

example

http://photos.nerail.org/showpic/?20050 ... ODE=&POYE=

Good evening,
Those inside rails are called "safety rails". They are commonly found on bridges. Their purpose is to prevent derailed cars from going over the side of the bridge.
Leverett Fernald

Those rails are known as "guard rails" which are placed at locations where derailments would have drastic consequences.The most common location is on bridges as is shown in the photo you have provided a link to.They hopefully prevent a car whose wheels have derailed from totally leaving the track which on a bridge would could lead to severe damage or injury.

I see. Thanks!

Neman is correct in that they are "guard rails". Some folks confuse restraining rails as guard rails, but the two are different. Restraining rails just allow for the general width of a flangeway from the travelling rails so as to tightly hold an axle on a properly railed car in place, where guard rails may spread over a few feet, in more of an effort to "catch" an already derailed car. Guard rails on Boston's El consisted of large timber which generally worked pretty well. (I'll have to forget my signature this one time !)