Once the last one dies, fall back to the railroad tracks and run up to the Russian commander. It's finally time to take the trestle bridge. Advance and try to stay low. There are several machine gun nests covering the entrance to the bridge. Try to stick to the left side and pick off the gunner. Then get to those sandbags and hold your ground. If you can take out the gunners, then the rest of the Russian force should be able to get close and support you.

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Just leapfrog from position to position and keep firing on the machine gunners. Once you get to the middle, you should be able to swing a bit to the right and take out the few guards by the marked building. Head inside and go up to the shells to trigger a scene with Dexter. Now that you have your explosives, you are ready to go.
Push forward and onto the bridge with the general Russian charge. Take out the first machine gun nest to the left before you get too far onto the bridge. Then jump over the sandbags and crouch. Plant the first pack of C4 here. Lean out and snipe the line of men at the end of the bridge. Then move up and curve over to the marked spot by the second support. Plant your C4 again. Rush over to the final spot and crouch. Don't worry about the machine gun nest on the bank. Just plant your charges. Some allied support will destroy them and the main German defensive line.
Once you regain control, just sprint off of the bridge and down to the left. Get to the marker to get clear of the German forces. After a little bit, you can turn around and watch the bridge explode. Until that happens, just move down to the farm.