From the trees near the start I could take out the first sniper in the right tower. Then from the centre of the rail track, between the two concrete walls that go up to the start of the span over the river itself, I could take out the sniper in the second tower on the left without being seen or heard. From that tower on the left I could take out the sniper directly opposite on the tower to the left (opposite side of bridge). For the last tower on the right (opposite side of the bridge) I had to get into the tower and kill him myself right at the top.
Hope that helps. It is a difficult mission (I set the alarm off twice and they never found me) but this really is one of the best missions in the game for freedom of movement compared to the missions inside buildings. There are lots of ways to complete this mission and mine is not necessarily the best way. Have fun
