Doubts with cargo deliveries. . .

I have multiple trains, and few construction type cargo compartments, but why can't we see a full list of what available items we can deliver via this ?

Are some of the items locked to certain trains ?

Eg, I have a few wrought iron, and have previously dispatched it via Flying sim, but not able to see wrought iron on Llama line or Simeo plus B, same casw with the material cars, where I'm not able to select electrical components or animal feed from every train where material car is attached.

Simeo plus B not showing wrought iron even when it was available.

Flying sim allowing wrought iron for delivery.

However, I noticed that if any of the delivery slot had a material or commercial item request, toggling between the slots is cycling between some of the available (specific car supported) items. 

Are we supposed to find and memorize by trial and error which trains show/allow specific items when trying to do a delivery dispatch ? Or am I missing any other hidden mechanic that could allow me to cycle between the available items. 

suggestion: to display what items will be available for dispatch while configuring the cargo compartments on any given train, before selecting and committing to dispatching the train