Do you not think that French rail has generated at least 50 billion for the economy? After Japan, France is the highest user of rail (km per person per year).
It's fine for public transportation to not make back the investment if the advantages to the public are high enough (though with the Franc that's easier to deal with than the Euro)
You can't apply that to infrastructure spending. Passenger and freight rail improves the overall efficiency of an economy, it's absolutely reasonable to attempt to determine the net benefit from it.
Given that the French economy has been stagnent for the last 15+ years and given that many countries seem to be doing well or even better without such a money wasting machine of a rail system, allow me to think that leaving those 50+ and growing billions euros in the pocket of tax payers would have been nicer and better spent. Besides, I never use trains for anything crucial because they are always on strike, happened to me once for an important meeting, never using it again. Also, because of our communist unions, they also close early at night so it's useless to go visit friends and come back at night, still need to get an Uber.
It's fine for public transportation to not make back the investment if the advantages to the public are high enough (though with the Franc that's easier to deal with than the Euro)