I had a lot of trouble with this infernal train level. Most of the game was relatively straight forward but that train owned me so many times.
1: Farm Money – To start, you need around $1000. You can definitely make do with less money but $1,000 will net you enough money to buy enough molotovs and syringes as well as get you the “Plunder Economy” achievement. To farm that money, an incredibly easy way is to farm the vehicle combat encounters from driving around on the world map. You can easily beat the convoys by pressing the button to engage in close quarters vehicle combat. Each convoy wiped should net you anywhere from $100 to $200.
2: Buy a lot of molotovs and syringes – After you farm some money, you can stop by any shop and purchase molotovs and syringes. I originally had 19 molotovs and 13 syringes, but you can buy more molotovs and less syringes. Shops are located at the start of each zone or towards the north of the crashed helicopter in the main zone.
3: Clear the level with sniper rifles – Clear the level by using at least one sniper rifle and kiting enemies over one at a time. Once all the areas leading up to the train are clear, you are safe to shoot the middle train section.
4: Throw molotovs – Once the middle section explodes and the bomb is exposed, throw two molotovs to clear each side car of enemy troops. Run up and have one person take each gunner seat and one person arm the bomb in the middle. Use syringes when needed and throw one molotov at a time at the incoming reinforcements. That should net you an easy victory!