Granite City received ore from a variety of sources and over various roads. On the M&StL list we have discussed ore traffic that moved on the M&StL to Granite City, via the CB&Q. Below are some messages from the list from about 10 years ago.
My understanding of the ore trains on the M&StL is the following: The ore came from a mine in SE Minnesota, near Rochester. No where near DMIR territory. This mine was on a CGW branch. The CGW transported the ore in standard 2 and 3 bay hoppers, even boxcars, but to my knowledge no ore jennies. The CGW used almost anything they could to move ore (we have a list of cars used). Some ore was handed to the M&StL at Marshalltown. The M&StL then moved the ore to the CB&Q at Monmouth. The CB&Q then delivered the ore to Granite City steel, near E St Louis. DMIR ore jennies seldom got off line, and if they did it was as a solid train direct to the mill in winter and back to the DMIR, no straying. DMIR ore jennies where not used in interchange as they had their brake line connections up high on the car body, not located next to the coupler as was standard practice. Unless you had a transition car or engine so equipped from the DMIR, you had no brake line on a stray ore jennie. Not a practical situation with a heavy commodity like ore. There has been a recent discussion about these ore trains on the CGW list. Some of the ore went towards Chicago and was routed to Granite City without the M&StL being involved.
Don Hofsommer's book (Tootin Louie) says on page 262: "New traffic such as iron ore loaded on Milwaukee Road in southeastern Minnesota and bound for steel mills in Granite City, Illinois (via M&StL at Mason City and Wabash at Albia)."The time period seems to have been the mid-1950's.
The ore that travelled over the old M&StL in the mid to late 60's was Missabe Range ore in ore jennies headed for Granite City. Even into the mid seventies these movements traveled the old StL as far as Marshalltown and then headed east for Nelson before turning south to Granite City.