An iron base plate is part of making rail tracks within the Artisans Workshop. To make them, an Iron ingot i can be used on an anvil, requiring 19 Smithing and giving 5.1 experience. They can be deposited as a raw component into the nearby rail cart, or combined with iron rails to create an iron track 40%.
To make the full railtrack, you need to make some iron spikes, and use them on the anvil to make the 60% track.
To make a 80% railtrack make some iron joints, and use them on the anvil too, to make 80% railtrack.
to make a full Railtrack -100%- you need to make some iron Tie, and use them on the anvil too.