[fabianfred]

If the map-maker has used things from the objects menu to lay down a bridge instead of using the proper bridge...then they will not work properly. Default bridges can only be used if the river is at sea-level and if the terrain is high this will cause the rivers to be at the bottom of valleys or canyons. The keep it natural sometimes the map builder chooses to ignore that and so is unable to use the default bridges.
Bridges built from objects will be ignored by the auto road/rail process which lays waypoints for trains and vehicles to follow a road rail/line.