I had a very short and consistent scheme for a while. No text, just icons.
{materiel icon}{chest icon}
For example, one station provides iron plates to the rail network (loads onto train), the other requests crude oil from the rail network (unloads from train):
Downsides: Some uncommon icons can be hard to read. Some liquids are similar in color and hard to identify when seen alone.
So later we changed to a better readable format. Examples:
[L] Iron Plates
[U] Crude Oil