Copperhead Rail Yard

Liberating this outpost also unlocks a good MG-mounted truck that you can deploy from any garage. The railyard itself has MG emplacements so going loud is a bad idea. It's better to approach the outpost from behind where there's a lot of visual cover. There's a lone sniper on the roof that you can snipe from below, as long as you entered the building first.

Once the sniper is dealt with, you can now proceed in picking off the scattered guards around the compound. Save the last two cultists along the tracks. As long as you cleared the other enemies behind them, you'll have a lot of space to hide around and ambush them without even knowing your position.