When getting started with RTT the amount of crappy documentation I wrote may seem a bit overwhelming. This post is here to try and give a quick getting started guide so you can have your fancy cargo trains up and running in no time.

In this case we'll be configuring a train to select its trailers based on the cargo it carries from a premade list of trailers.

1. Make sure you have a bit of a variety in train assets so the mod can use the trailers. There are also some packs that contain just trailer assets out there, those are great for RTT as well. I recommend getting assets from THIS COLLECTION as the mod already has some premade configs for them!
   
2. Load up a game and open the mod's panel. Look for a slightly stretched train icon somewhere on the right of your main menu bar.

In the window that opens, click on the Add New button to create settings for a train of your choosing. A search window will pop up. Go ahead and find your cargo train in there and click select.

Now you should have returned to the main window and it will show you the settings for the cargo train. Next are the following steps:

1. Uncheck "Use default trailers". We are using a premade list of trailers, so we don't want the trains original trailers to interfere with it.
   
2. Check "Cargo changes trailers". This is what toggles the selection of trailers based on their cargo types so make sure it's on!
   
3. Click Add Collection.

Now you should see the search window again. This time it will show a bunch of trailer collections. Trailer collections are, well, collections containing trailers (and their settings) that the mod can pick trailers from. There are a bunch included in the mod itself that I recommend using for starters since you won't have to go trough the effort of making one yourself. These collections make use of trailers in the collection linked earlier. Don't worry about subscribing to all assets in there, if one is missing the mod simply won't try to use the missing trailers.

The following collections are part of the mod:

• UK Freightliner Intermodal Loaded
  Contains the trailers of my Class 66 with container cars, nothing else.
  
• Generic Containers EU
  Contains all the EU type generic container trailers from the (short) trains found in this collection:
  http://steamcommunity.com/workshop/filedetails/?id=854338073
  
• Mixed Cargo EU Closed Goods Wagons
  A collection of european cargo wagons. Uses the closed goods wagon packs in the collection.
  
• Mixed Cargo EU Generic Containers
  A collection of european cargo wagons. Instead of closed goods wagons this uses the generic container wagons that are in the Generic Containers EU collection as well.
  
• Mixed Cargo EU Real Life Containers
  A collection of european cargo wagons. Goods and Food use ron_fu-ta's container wagons.
  
• Mixed Cargo Scandinavia
  Contains only Von Roth's scandinavian train assets. I recommend using this in combination with said assets (engines) since they seem to be scaled down a little compared to other train assets.
  
• Generic Containers US
  Contains all (current) container cars from bsquiklehausen. Also includes the ones from the FCC super long series.
  
• Mixed Cargo US Boxcars
  Contains most bsquiklehausen cargo wagons, no containers, uses boxcars instead.
  
• Mixed Cargo US Containers
  Contains most bsquiklehausen cargo wagons, no boxcars, uses containers instead.

Pick one you think would fit the train you are currently editing and hit select to select it. If you want to add multiple collections you can repeat the "Add Collection" process for all of them that you want to add. Note that the mod will pick a collection at random and then pick trailers from that collection only! It doesn't mix collections!

With your collections in place you are done! Hit the save button to save and apply the config and watch your trains get randomized. If you want other train assets to change as well you'll have to configure them too of course, the same way you did this one.

For some more control or to use trailers that aren't in the default collections you will have to manually assign the cargo types for each wagon. This can be done for both collections and for trailers that are only used in a single train. Check out this imgur album[imgur.com] for an example of how to do that.

Note: you do not need a wagon of every cargo type in your train. If the required type is unavailable the mod will use the "next best thing" depending on what is available.

For more info on all the options, check out THIS discussion page. It contains info on the actual config file format, but you can find all (most?) settings in the UI as well.