Unloadable Train Wagon

Spawning properties:

Anything train-related spawns random wagons, usually with a high chance of spawning at least one engine-type train car, on temporary branch-off points neighboring the main track.
The way spawning works is that the server checks for wagon population, which appears to be around 90 wagons, every minute or so. New wagons can only spawn when other wagons are somehow removed from existence, reducing the wagon population in the server. The server does not care how close a player is to the track when this spawning happens, but it does have a very high chance of spawning new wagons on the least populated neighboring tracks first.
Please note that adding multiple wagons to a branch can make that branch never spawn new wagons until the branch is clear.

Types of wagons:

• Work cart (first seen in tunnels)
• Armored work cart
• Red/black amtrac caboose (high priority engine)
• Flatbed (not useful)
• Red transport wagon (not useful)
• Green transport wagon (not useful
• Gambling casino wagon (not useful)
• Unloadable loot wagon (has 3-5 crates, randomly either normal crate, med-crate, or mil crate)
• & Unloadable resource wagons (100% what you want)

Okay, but how do I get the useful ones?

On every branch-off point on the main track is between 1 & 7 wagons of any kind, likely with at least one random engine, with the rest having seemingly equal pot of spawning.
Currently if there are no good wagons on the map, you may need to wait for the wagons to despawn, usually by pulling them on the main track and letting them sit for half an hour. However if they are ever moved then the despawn timer may reset.
Wagons seem to have trouble despawning if they are sitting in the track in/around Train Yard, especially engines, so get them elsewhere after you're done if you plan to keep using the trains for resource gains.
As of now, the optimal method of killing a wagon efficiently is either one HV rocket & 1 grenade (totaling to 260 sulfur used), or 8 grenades (totaling to 480 sulfur). All but engines & casino have 750 health. HVs do 660, grenades do 100, explo does 10 each.
The most efficient way of removing wagons is to HV between two connected wagons, and then laying a grenade between them right after, halving your sulfur use. However, I found that if you can get two sets of two useless wagons neighboring each other (one set on the main and the other set on a very very close branch, the explosion radius of both the HV and grenade will still be enough to kill all four wagons, QUARTERING the sulfur required to gamble for resource wagons.
Do note that because Casino is 1200 health instead of 750, it will take 2 HVs instead of 1 & 1 grenade.
Some of the best ways to scout trains are minicopters or horses, a pro and con for each. Horses are good for fuel economy and you can check loot wagons for mil crates with HQM/guns but you are forced to stop and check amounts on resources and what resource type. Minicopters can make a single glance at the contents of a branch, and can see from above a visual of how filled up the resource wagons are, but will not be able to check loot wagons.
If a resource wagon looks half full, it is filled at or below the halfway-point between its min & max amount. If it looks pretty full, it's over that halfway point.

What do you need to do to get the loot from these?

• One green card
• One fuse per 4-5 unloadable wagons
• No puzzle, just swipe the card

Take your train to the Train Yard monument and bring it beneath the extractor building the tracks go beneath, and stop the train. Swiping the green card in the door beside where the train stops will open the door for a long amount of time. Note that there is a similar door up top that few players know about with a similar green card door. Both doors access the fuse room where it will automatically move the train without using fuel by pressing the Forward or Backward buttons at the console, and extract unloadable wagons with the Extract button.
Here is a link visually explaining the basics of doing this event: https://youtu.be/a30ETOIV4Ss
All train engines take the same amount of fuel; 4.5 low-grade per minute. They are all extremely fast for the fuel they use, but the red amtrac caboose is notably faster than the work-cart or its armored variant, and can pull almost twice the number of wagons before the load makes it impossible to move.
There is no limit to how many engines or wagons can be attached at once, so as long as you have enough fuel and engines, you can have a line of wagons that quite literally span the map's entire main track.