I probably don't understand how the game decides which cargo to load when there's more than one option. However, my Steel Mill at (423,153) has plenty of potential customers waiting for steel and thousands of tons of iron ore and coal waiting at the next stop (and destined for the mill). But the trains pick up iron ore exclusively, with 14,998 tons in storage at the factory already. Not a grain of coal, for years now.
Secondly, when the trains reach the station, the iron ore now simply disappears (apparently since I switched to 11.17 today; it seems to have been stored in 11.16). It is neither consumed nor added to storage. I receive no income from the transport. The Gasworks next door seems uninvolved. Yet the stop counts the ore as Arrived.
http://freakshare.com/files/ohpn99jt/1.rar.html
Hmm - I have not changed this code from Standard: I suspect that it is a very simple system for working out which cargo should be loaded, and does not take into account whether the receiving industry already has enough of the cargo in question or needs some other cargo.
The issue here seems ultimately to be that there is far, far too much of both types of cargo en route to the steel mill. This ultimately comes from having a very high "maximum_intransit_percentage". I notice that you have not manually changed your "maximum_intransit_percentage" down from 16,000 to the new recommended level of 110 (from 11.17 onwards). You can do this by pressing the "i" key and going to the "economy" tab. Once you do this, you will need to wait for the current backlog to clear before you will notice any difference in behaviour, but things should improve thereafter.
Thanks! I forgot about the "I"-key over the years (I looked for an "Options" button in the menu, though), and I missed the bit about manually reducing max_intransit.
BTW, thanks for all the hours you put into this!