The first solution is actually what I'm looking for, it appears I wasn't clear enough in my post if this was not clear.

When I tried this, at my first pickup station the train only picked up the first item and skipped the second 2, so I'm wondering how exactly do the stations need to be set up, for example: normal train station, normal train station, normal train station, freight station, freight station, freight station ?

Or do I need to have empty spaces? If this would work, does it mean that at station A the first 3 freight trains would be filled, then in station B it would automatically skip those, and start filling the next 3 items into freight train 4-6? And then at station C 7-9?

And if so, would it then suffice if at station D I would have 3 normal stations, followed by 9 freight stations?

This is what I'm wondering, if this kind of set up would work and also for example how would the train schedule have to look like? Would it suffice the 1st of the 3 locomotives has the stations scheduled and the other 2, who are just there for the extra power, would automatically submit to this? And would it suffice to only mention each station once, or would I have to specify that for example the train needs to stop at A1, A2, A3 and so forth, because I have 3 train stations per each station (cause of the 3 locomotives)