Such a railroad would allow us to directly ship goods from one end of the continent to the other overland, bypassing the Babkhan-controlled Dyre Straits and sometimes-hostile Republic of Antica. It would also help bring commerce to the underdeveloped areas of inner Kildare and west Antica, and have immense military value.
If we end up giving up Cognito because of the problems with the MCS, we can cut back on the scale of the project and have the eastern terminal at Liminas or one of the Kaikian cities.
Here's a sketch:

