TL;DR: New type of train car, can only carry raw resources, such as ores. It would have more capacity than a regular cargo car. New entity, can load/unload items from this car very quickly.

*Note: I use the word loader entity, but I am not referring to the loader in FFF #128 . This is similar, but ...

Did you film actual Factorio footage in the trailer? If so, how many tries (and extra helping code) did it take to get the timing to work out just right?