As you increase rail capacity you can get additional goods. There is an upper limit until additional raw goods are developed. But too low a capacity limits the amount of planks etc you get.
I've gotten planks etc at 4 but never finished goods.
I have gotten additional Finished goods by upping the Finished Goods to 8 without an equal increase in the raw goods (e g 4 trees to planks)

It's a shame that Imp1 has such a ridiculous amount of hidden information. I think 80% of it is hands down better than Imp2, but sometimes I fire up Imp2 instead just because I want some solid, up front mechanics.