It's called a 'Check Rail'. Usually you can see these on the inner side of curves. If a train derails on a curve, it will tend to move over the outer rail ( The wheel will climb over the outer edge of the curve). In that case, the inner wheel will touch this check rail and stop further outward movement. You can see these kinds of rails at points and switches also, where trains switch tracks.
Both terms are interchanged frequently. But normally, the term 'Guard rails' is used for the cases where there are 2 inner rails. These are usually seen on bridges and belo FOB's etc. The 'term 'check rail' is usually used for the cases like in the above pic, where the extra rail is on only one side of the tracks. However, there is no fixed definition for these terms I guess and I have seen people use both terms to refer to such track layouts.
Sep 23 2016 (2:02PM) Loco Tag: ET/WAP-4/22709 added by Deependra Singh~/872188
Lokmanya Tilak Terminus Howrah Samarsata Superfast Express With Offlink ET WAP-4 In Lead Watch It On YouTube:click here _______________________________________________ 12151 Lokmanya Tilak Terminus Howrah Junction Samarsata Superfast Express Led By Offlink ET WAP-4#22709 Skips Bilaspur Outer Through Main Line For Having A Before Time Entry At Bilaspur Junction.