Letter to Burney is a written note found in Dishonored.
I think I have what you're looking for. A rail car containing blueprints for an advanced ammo type was lost in the flooded district. The fools who designed it thought it so valuable that they sent the only copy along the elevated lines running through that abandoned place. It never reached its destination. I suggest you start your search at Rudshore Central Rail Station.
Be bloody careful - rumor has it that the Whaler Gang has their territory somewhere in that area. If you see any sign of them, run like mad.
It can be found during the mission The Flooded District in the Rudshore Waterfront, across the street from the entrance to the Greaves Refinery. It only appears if Corvo Attano did not take the explosive bullet blueprint in Dunwall Tower during the mission, Return to the Tower.