Slightly north of the train tracks, north of the Bus Stop, you'll notice a large tree of sorts (not sure what this species is called - perhaps it's a Baoab tree?). However, next to this tree you can find a suitcase sitting on the ground.
This briefcase is located on top of one of the train cars at PetroSahel. To get to it, you can climb on top of the closest fuel silo; if you sprint towards the edge and spam the jump key, you should be able to land on top of the car. Be sure to quicksave before attempting this as failing to land on the car is likely to get you severely injured or even killed.
This briefcase is sitting on top of a very large rock, north of North Railyard. To reach it, you'll need a vehicle - I tend to bring an Assault Truck myself. Park the vehicle on the western side of the rock as depicted in the smaller image in the screenshot. Jump onto the vehicle and onto the highest point on it; face the rock and get as close as possible without falling off the vehicle. Now move towards the rock and spam the jump button. If you're in the right position you should be able to scale the rock and find yourself on top. If you slide off, try a slightly different place to jump up the rock.
This briefcase is located on the roof of a train car inside the largest building at the North Railyard. To get to it, go outside the buidling on the southern side - there you should spot a ramp leading onto the roof. Move up and look through the holes in the roof; once you spot the briefcase, drop down through the roof and loot it.