It can be found at the flooded trainyard, on a bed in the blocked-off warehouse, accessible via the walkway between the two warehouses.
Squire Schultz: Scouting report. Brotherhood Squire Schultz. This whole trainyard is half-sunk in the ground. Anyone in the middle of that mess is just begging for a sniper to pick them off. These two warehouses seem to be the only good defensible position. There's a nice walkway between them and plenty of places to get the drop on someone. Heh. I might actually be getting the hang of this.