Trade
Iron allowed more trade opportunities with the mass, distance and amount of good that needs to be traded. It also increased productivity.
Iron allowed travel by bridge and by rail roads. Cast iron allowed for bridges to improve because of its stability and because of its ability to hold a lot of weight. Iron allowed the railroads to be built. Iron allowed bridges to become stronger. It allowed more goods to be traded. Whether it be food, luxury goods or things like wood. Trains allowed easier transportation because it could take multiple items of one thing, or multiple items of many things. Lumber is a big item and is a lot of work to transport. But with a train and some machinery, it can be loaded and shipped easily. But without bridges, no matter what it is, the good cant be shipped. Bridges allowed goods to get to there destination quicker. Without the bridge, what ever is transporting the good would have to go around the stream, river, or small lake. But with a bridge, the transportation doesn't have to stop and transfer the goods or go around. The transportation can go right over it without wasting any time. Bridges and railroads allowed for goods being traded to be more officiant.