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I am assuming the box disappeared on the side you were loading. Check the other side of the bridge and see if the box is there. If so You will have to take the 25k to that end of the bridge. I did the same thing, the trigger and the box disappeared and the bridge never finished. Although the box was still there on the other side of the bridge. I built the bridges that would get me to the other side and then dropped the materials in the box and the bridge completed. If you haven't already done so I would highly recommend building a Construction Storage building and to store your material. It creates a cargo container that holds 25k of that material making it easier to build bridges and other buildings.