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.