Extra set of rails on bridges

[At_traction]

After going through 16 pages' worth of search results and apparently with no related threads (or perhaps the search terms were off, go figure):

I've long wondered what is the extra set of tracks at (some) bridges atop sleepers, in the space between running rails.

Is it to somehow stabilize and "rigidize" the track at such points of discontinuity as bridges? In case of bridges, is it mainly used for bridges with the sleepers laid atop stone ballast, ie. does the method of track support play a part in choosing the placement of these extra rails?

Thanks.

 

[PaulLothian]

To minimise structural and vehicle damage in case of a derailment. Intended to keep train within structure gauge at that point.

 

[pendolino]

Four rails so groups of four small children can have a rail each to walk along in a 'coming-of-age' film. Having two groups of two wouldn't be so photogenic.

 

[At_traction]

To minimise structural and vehicle damage in case of a derailment. Intended to keep train within structure gauge at that point.

Thanks. That makes sense.