If I build a freight rail terminal in my trade port, does the rail terminal provide for more freight added to the one already delivered by road, or does it replace the road deliveries entirely by train deliveries?
Adding a Rail Terminal or Ship Dock will not replace your Trucks. The Trade Port was not designed to function like this, but if you delete your exporting/importing truck bays, all of that work will be offloaded to trains or ships. Read this if you're interested in doing that.
It simply provides more frequency of your cargo importing/exporting. There is a limit to the amount of resources trucks can handle (not to mention traffic problems), so adding a Rail Terminal or Ship Dock to your Trade Port will greatly increase the amount you're able to export or import.
enorl76 is correct in stating goods move intracity via trucks.
The trade depot will attempt to resolve the need via network type, so when you add a freight terminal, then you can additionally ship out via rail. In "real life", rail is significantly more efficient than by road, but rail doesn't go "the last mile".
Trucks will still be used to ship to destinations that request the resource within the city.
When you enter construction mode, you can see on the right, "Road Sea Rail". When you hover over with the mouse, you can see the amount of traffic via each network type.