Railroad viaduct over the rio toro near Salta (Argentina).
On the viaduct there are two pairs of guard rails, so that in case a train car derails while on the bridge, it will likely still be on rails, to prevent a deadly accident.
For more info about railroad guard rails, go to en.wikipedia.org/wiki/Guard_rails_(railroad).
This train line between Salta and San Antonio de los Cobres (Argentina) is sometimes called "Tren a las nubes". For more information, read en.wikipedia.org/wiki/Tren_a_las_Nubes