not a big issue as the height of coaster/van is not more than shipping containers, so clearance under bridges is not an issue but the driver'd be much happier to drive that van off instead of that trailer from KHI to lets say ISB.
2ndly trailer carrying container is more difficult to drive e.g. in heavy cross winds whereas this van can let wind pass underneath making an easier job. also its springy in nature thus accomodating weight distr. better than container.
steel rods could be carried in other ways too, actually this situation accounts for the greedy nature of transporters, transporting two different goods/order in one go by saving cost of sending another truck for such purposes.