T'was a dark and stormy night...
This mission is to recover a captured alien artifact, traveling through a dairy farm to where a train carrying the device derailed. Approaching the farm, a drone will speed by and go into the distance. If you've got a sniper rife or you have a Recon with you, your first target should be the Rocket Sniper up in the barn's loft. After which, there will be a bunch of Sectoids. Taking them out, Outsider grunts will pop in both from a drop pod, and on foot, from across the yard.
Head on through to the milk shed, there's a few silacoids around in here. The Laser Pistol can be picked up in the room to the left. Exiting, there's a supply drop near a well and a river. Head into the water, where there'll be a few more silacoids. One will knock a flatbed railcar into the river, and you can get up onto (relatively) dry ground again. Tag the fleeing silacoid. In that first railcar you can go through the Gunner Pack schematics is on the floor, hard to miss.
In the first trainwreck area, it is only grunts. Eventually, some more grunts and a drone breaks open the next 'door' traincar. At any time, another traincar that's already open has the Laser SMG and energy packs in it for you to get-- in fact, if you don't have Carter with the Scatter Laser, you may want to take it now. Then go to the next area.
You'll see the device, but a drone will steal it and take it past the next railcar-- when the car's door buckles, again, and a Muton punches his way out. He'll be supported by the drones, so have your agents focus on the drones while you start on the Muton's armor. He's stuck in the punch animation for a good bit after opening the door, and the mission is almost over, so feed him all your grenades. If you stay in your side of the flatbed car in front of you, the Muton will often go the long way around the crate car to your right, so be ready to move your agents when he comes around. As hinted at: the Scatter Laser works well against him if he gets close, and the SMG is good at a range (either or, both do more damage than ballistic guns or the Laser Pistol).