So I tested this in a new map, connecting up an ore mine and coal mine with zero production to a steelworks with zero production. In this case the coal mine was right across the map from the steelworks.
It took a while, but the production at the coal and ore mine slowly ramped up and is now at a good level and using my lines to deliver to the steelworks. It’s been a total money sink though. The lines have run at a (large) loss for about four to five years and are only now looking like they might start to turn a profit.
I had one unintended consequence. I hooked up a rail line to a town from the steelworks to deliver the goods, but it decided that a nearer town by road was more attractive so if I want to capture that trade I’ll have to either run more trains on the line or put in a road route to that town (it’s too close and uphill for rail).
There’s a lot of depth here in working out how to hook things up for the maximum benefit, and with industry free to decide another option is more attractive there are few sure things.