Really, I think it means very little. The expense of working (let alone building) a railway through a shifting desert, with water hungry locomotives would make shipping costs even more unfavorable vis-à-vis ocean shipping than already existed. Internal combustion motive power was not sufficiently developed at this time to be a viable alternative, and the cost (capital & supplying) of electrification would have driven the price tag to national-bankruptcy levels.
Most trade between France and her colonies will continue to be by sea, and the railway will become a little used money sink. I could see most of its traffic being simply its own supplies.
After doing a quick google search, this was an idea that floated about prior to 1900, and serious consideration was given to it.
The project began to take form in the 1870s after the end of French conquest of Kabylia, though there would be to wait until the 1890s to see a study commissionned by the government on its feasibility before fading into obscurity as noone bothered to invest in it. The first real attempt to build it was started by the Vichy regime, but quickly aborted due to the wartime context.
https://www.globalsecurity.org/military/world/europe/fr-trans-saharan-railway.htm
I imagine no ww2 and a longer lasting colonial presence would eventually lead to this attempt being undertaken more seriously.
Technically, the project was very feasible by the time technology. Actually, the reports advanced that the main difficulties would be building artworks, tunnels and bridges across the Atlas mountains to connect the Mediterranean coast with the Sahara interior where a flat land, not all the region is made of ergs (sea of dunes) but also of regs, or flat and rocky deserts, which make laying railroad pretty easy (not minding the temperature levels to work under) with estimated rate of 1 km per day, and as of water, it's largely available provided wells are drilled into the fossil aquifers of the region.
Eventually, the Transsaharan route combined to the Marseille-Algiers shipping lane has the strategical advantage of shortening a lot the route to French colonies in West Africa and a much less exposed one as the Western Mediterranean is secure for the French navy, one of the most powerful navies of the world before ww2, well ahead of Italy, without counting British support.
Plus, water drilling would be probably leading up to an earlier development of Saharan oil, and make the railroad even more crucial infrastructure to support oil drilling in the region.