Members of two more Iowa National Guard units are being mobilized for duty, but they won’t be going any farther than Illinois. Railroad bridges over the Mississippi River at Clinton and Fort Madison are now under 24-hour watch as they’re considered critical assets. The Iowa bridges are among 250 sites nationwide singled out last week as needing tighter security. The Guard has assigned the 224th Engineer Battalion, based in Fairfield, to guard the Fort Madison bridge, while the 834th Engineer Company out of Davenport will patrol the bridge at Clinton. A total of eight soldiers will be involved in the effort, assisting local and state law officers as part of what’s being called Operation Liberty Shield.