Suppose, for a moment, that there is a human inhabited supercontinent. The planet that this supercontinent is on does not have any large landmasses which are more than 10 KM from the primary continent (at the closest point). Realistically, there may be some islands a bit further away; assume for the time being that they are of only minor economic importance.
The perimeter of the supercontinent is roughly 40,000 km in length. Assume it is roughly circular for the time being.
The total land area is similar to that on earth, but due to it's arrangement, considerably less of the available land is habitable to humans.
Since it is difficult for rain to travel across vast distances, the interior of this super continent will be a massive desert, meaning human habitation will be concentrated on the perimeter of the continent.
For much of history, shipping and travel would be done via boat, sailing on the supercontinent's edge; it's probably faster than driving horse carts around the edge of the continent.
Now, suppose someone builds a large railroad which spans the entire perimeter of the supercontinent. In addition, rail bridges or underground tunnels are built to reach any landmasses close enough to the main body of the continent for the construction to be feasible. This railroad has at least four lines that all completely encircle the supercontinent; two or more in each direction.
This railroad will be built during the steam era; it will initially not be possible to build long routes through the interior desert, but as technology improves, it will be possible to build longer and longer “shortcuts” through the arid continent interior.
In this case, are oceangoing ships now doomed to be used only to ferry goods and passengers to and from the various small islands that dot the edge of the supercontinent? Or are ocean going ships still competitively viable on routes from one part of the supercontinent to another?
Bonus: In a scenario like this, is it economically feasible that airplanes or airships will become common, or will economics force them to be a niche form of transportation, used only by the ultra wealthy?
does this calculation change if, later in this civilization's history, they manage to build railroad tracks cutting through the center of the interior desert, meaning that trains traveling from one side of the continent to the other can take the direct route rather than going all the way around the perimeter of the supercontinent?