How can I force ONLY rail export of goods from a trade port?
To force a trade port to only deliver goods using rail connections, simply destroy all of the delivery trucks for that trade port. This can be done by using the bulldozer to destroy the delivery truck garage on the edit building screen.
The suppliers of your trade goods will still be delivered by the building that creates them using their own trucks, but any good that arrive at the trade port will only leave via rail (every 3 in game hours).